METZO AM BROADCAST TRANSMITTER WITH AUDIO COMPRESSOR Frequency synthesized, using a crystal oscillator for drift free operation and accuracy Page 1
Table of Contents INTRODUCTION 3 QUICK SET-UP 4 Opening the Case 4 Selecting Operating Frequency 4 Select the part of the broadcast band you are going to use 4 Frequency Selection Chart 5 Ground 8 Antenna 8 UK 8 US 8 Audio Input 8 Audio 8 Marker Tone 8 ATU Selection 9 Power 9 Fine Tuning 9 Audio Level Monitor 9 Operating Controls 9 OPERATION 9 Front Panel Controls 9 Gain the Blue Knob 9 Compression the Yellow Knob 10 Modulation / Rotation Point (Limiter) - the Red Knob 10 INTERNAL CONTROLS 10 Audio 10 Stereo to Mono Mixer 10 High Pass Filter (Internally Adjustable) 10 Low Pass Filter (Internally Adjustable) 11 Compressor / Limiter 11 Noise Gate (Internally Adjustable) 11 Level Monitor 11 Audio Level Monitor (the default setting) 11 Audio Level / Modulation Monitor 11 Antenna Tuning Indicator (for use with External ATU setting only) 12 Frequency Synthesizer 12 Antenna Tuning Unit 12 Introduction 12 ATU Operating Mode 12 Internal 12 External 12 APPENDIX A BLOCK DIAGRAM 13 APPENDIX B PCB LAYOUT 14 APPENDIX C - CONVERT METRES TO FREQUENCY 14 APPENDIX D GROUND CONNECTIONS 14 APPENDIX E - AERIAL EXPERIMENT 15 Some Suggestions 15 Aerial Tuner and Loading Coil 15 Loading Coils 15 APPENDIX F ATU SETTINGS 16 FAQ 16 SPECIFICATIONS 17 SIMPLE WARRANTY 17 Page 2
Introduction Play music from your CD player, Tape, FM radio, MP3 or Old Time Radio cassettes on your favourite vintage radios! With the Metzo AM Transmitter, it is easy to transmit high quality audio signals to any AM broadcast band radio receiver between 500 and 1800 khz. (UK Medium Wave / US Broadcast Band) Compatible with European or American transmission using 9 KHz and 10 KHz channel spacing. Accepts audio from any Mono or Stereo, with internal Stereo to Mono mixer Internal Audio Compressor and Limiter Crystal Controlled Frequency Synthesiser using Phased Lock Loop (PLL) Internal Antenna Tuning Unit (ATU) with optional External ATU mode 6 IC and 4 Transistor LED Bar Graph Modulation Monitor and Tuning Indicator No FCC license is required, when used as directed it is perfectly legal under Part 15 of the FCC rules. (US Only) Note: Before using your transmitter, please read the sections dealing with installation and operating procedures. Front View Transmitter Gain Compression Rotation Point (Limiter) Rear View Antenna Ground / Power Left / Right Page 3
Quick Set-Up Your transmitter can be used right out of the box and is pre-set to: 1600 KHz - US Models using 10 KHz Channels or 1440 KHz - European Models using 9 KHz Channels You will probably want to change the Frequency, just follow these easy steps: Opening the Case Turn the transmitter upside down and remove the two screws. Turn the transmitter the right way up and gentle lift off the top cover. You can now access the switches and jumpers inside. Selecting Operating Frequency First you should fine a area of the medium wave (broadcast) band that is free of any stations and interference, this will vary from location to location the best way is to tune your AM radio from one end of the broadcast band to the other looking for suitable gaps. Notes: More stations will be heard at night so you may need to do this again in the evening. The lower half of the broadcast band is usually less congested. The upper half of the broadcast band will give you more range. Optimum range will usually be found from 1300 to 1700 KHz. Select the part of the broadcast band you are going to use Set the tuning switch SW-A and SW-B according to the look-up table, on the following page. Note: Your transmitter will be pre-set to 1600 KHz US / 1440 KHz UK Frequencies below 520 or above 1720 are not recommended as these are outside the allocated medium (broadcast) band, however the unit will usually tune below and above those frequencies stated. PCB showing position of Frequency Selection Switches SW A & B Page 4
Frequency Selection Chart For UK models with 9 KHz Channel spacing use Frequency in Column ONE For US models with 10 KHz Channel spacing use Frequency in Column TWO UK US Frequency Select Switch SW - A Frequency Select Switch SW - B Freq Freq 1 2 3 4 1 2 3 4 450 500 ON ON ON ON ON 459 510 ON ON ON ON ON 468 520 ON ON ON ON 477 530 ON ON ON ON ON 486 540 ON ON ON ON 495 550 ON ON ON ON 504 560 ON ON ON 513 570 ON ON ON ON ON 522 580 ON ON ON ON 531 590 ON ON ON ON 540 600 ON ON ON 549 610 ON ON ON ON 558 620 ON ON ON 567 630 ON ON ON 576 640 ON ON 585 650 ON ON ON ON ON ON ON 594 660 ON ON ON ON ON ON 603 670 ON ON ON ON ON ON 612 680 ON ON ON ON ON 621 690 ON ON ON ON ON ON 630 700 ON ON ON ON ON 639 710 ON ON ON ON ON 648 720 ON ON ON ON 657 730 ON ON ON ON ON ON 666 740 ON ON ON ON ON 675 750 ON ON ON ON ON 684 760 ON ON ON ON 693 770 ON ON ON ON ON 702 780 ON ON ON ON 711 790 ON ON ON ON 720 800 ON ON ON 729 810 ON ON ON ON ON ON 738 820 ON ON ON ON ON 747 830 ON ON ON ON ON 756 840 ON ON ON ON 765 850 ON ON ON ON ON 774 860 ON ON ON ON 783 870 ON ON ON ON 792 880 ON ON ON 801 890 ON ON ON ON ON 810 900 ON ON ON ON 819 910 ON ON ON ON 828 920 ON ON ON 837 930 ON ON ON ON 846 940 ON ON ON Page 5
UK US Frequency Select Switch SW - A Frequency Select Switch SW - B Freq Freq 1 2 3 4 1 2 3 4 855 950 ON ON ON 864 960 ON ON 873 970 ON ON ON ON ON ON 882 980 ON ON ON ON ON 891 990 ON ON ON ON ON 900 1000 ON ON ON ON 909 1010 ON ON ON ON ON 918 1020 ON ON ON ON 927 1030 ON ON ON ON 936 1040 ON ON ON 945 1050 ON ON ON ON ON 954 1060 ON ON ON ON 963 1070 ON ON ON ON 972 1080 ON ON ON 981 1090 ON ON ON ON 990 1100 ON ON ON 999 1110 ON ON ON 1008 1120 ON ON 1017 1130 ON ON ON ON ON 1026 1140 ON ON ON ON 1035 1150 ON ON ON ON 1044 1160 ON ON ON 1053 1170 ON ON ON ON 1062 1180 ON ON ON 1071 1190 ON ON ON 1080 1200 ON ON 1089 1210 ON ON ON ON 1098 1220 ON ON ON 1107 1230 ON ON ON 1116 1240 ON ON 1125 1250 ON ON ON 1134 1260 ON ON 1143 1270 ON ON 1152 1280 ON 1161 1290 ON ON ON ON ON ON ON 1170 1300 ON ON ON ON ON ON 1179 1310 ON ON ON ON ON ON 1188 1320 ON ON ON ON ON 1197 1330 ON ON ON ON ON ON 1206 1340 ON ON ON ON ON 1215 1350 ON ON ON ON ON 1224 1360 ON ON ON ON 1233 1370 ON ON ON ON ON ON 1242 1380 ON ON ON ON ON 1251 1390 ON ON ON ON ON 1260 1400 ON ON ON ON 1269 1410 ON ON ON ON ON 1278 1420 ON ON ON ON 1287 1430 ON ON ON ON Page 6
UK US Frequency Select Switch SW - A Frequency Select Switch SW - B Freq Freq 1 2 3 4 1 2 3 4 1296 1440 ON ON ON 1305 1450 ON ON ON ON ON ON 1314 1460 ON ON ON ON ON 1323 1470 ON ON ON ON ON 1332 1480 ON ON ON ON 1341 1490 ON ON ON ON ON 1350 1500 ON ON ON ON 1359 1510 ON ON ON ON 1368 1520 ON ON ON 1377 1530 ON ON ON ON ON 1386 1540 ON ON ON ON 1395 1550 ON ON ON ON 1404 1560 ON ON ON 1413 1570 ON ON ON ON 1422 1580 ON ON ON 1431 1590 ON ON ON 1440 1600 ON ON 1449 1610 ON ON ON ON ON ON 1458 1620 ON ON ON ON ON 1467 1630 ON ON ON ON ON 1476 1640 ON ON ON ON 1485 1650 ON ON ON ON ON 1494 1660 ON ON ON ON 1503 1670 ON ON ON ON 1512 1680 ON ON ON 1521 1690 ON ON ON ON ON 1530 1700 ON ON ON ON 1539 1710 ON ON ON ON 1548 1720 ON ON ON 1557 1730 ON ON ON ON 1566 1740 ON ON ON 1575 1750 ON ON ON 1584 1760 ON ON 1593 1770 ON ON ON ON ON 1602 1780 ON ON ON ON 1611 1790 ON ON ON ON 1620 1800 ON ON ON Note: Your transmitter will be pre-set to 1600 KHz US / 1440 KHz UK Page 7
Ground Connect the transmitter s GROUND / EARTH tag to a separate Ground connection. Whilst not absolutely necessary for short range (around the house) operation. You are advised to connecting a separate ground to the transmitters Ground terminal. If you do this you will get: A significant increase in the transmitters performance (more range) Significantly better Signal to Noise performance for the Audio stages NOTE: To avoid hum you are strongly advised to use the Ground connection Antenna Connect the transmitter to the Antenna. The transmitter has been designed to operate with the supplied 9 foot Antenna wire, which is adequate for local transmission. UK The Antenna wire should either be run out vertically for maximum range. Avoid proximity to nearby metal objects. If more range is needed, try relocating the unit to a different location. US The Antenna wire should either be run out vertically for maximum range or connected to a FCC Part 15 compliant External Antenna system. Avoid proximity to nearby metal objects. If more range is needed, try relocating the unit to a different location. Audio Input Connect the Audio lead to a suitable audio source, for example a CD player headphone output. The supplied cable can be used for both Mono and Stereo Audio Turn on your Audio source (for example a CD player), keep the volume low, otherwise you will get distorted audio from the transmitter. Marker Tone You can assist locating and tuning your transmitter by enabling the marker tone generator which will emit a constant tone to help locate you signal on a crowded band. It is also a useful aid when adjusting the Antenna trimmer capacitor for maximum range. You can select either a high or low tone depending on the location of the link TONE in position A or B, when not needed simply place the link on the centre pin only. Page 8
ATU Selection Your transmitter is pre-configured for operation using the Internal ATU in the band 1000 KHz to 1700 KHz (the most popular) if you have selected a frequency other than this please refer to APPENDIX F for details of how to reconfigure the ATU. Power Connect the regulated 15 volt power supply and switch on. Fine Tuning Next fine tune the signal using the ATU Trimmer capacitor, adjust for best audio quality and RANGE. Audio Level Monitor Best transmission will be achieved when the bar-graph is showing the Green LED is ON and the Yellow LED is pulsating strongly. Note: In the absence of any signal all the LED s will be off. Operating Controls Once set-up and the aerial tuned, the only control that needs be adjusted is the audio level if a different audio source is used, the unit will accommodate various sources of audio CD players, tape or cassette decks, PC or FM Radio. Operation Front Panel Controls This figure shows the suggested settings for the front panel controls and LED s Gain the Blue Knob NOTE: we suggest you leave this set to the mid-position This is the singly most important setting - to insure the optimum signal to noise ratio. Transmitter gain set to low gives poor signal to noise ratio The audio signal to noise ratio will never be any better than it is at the transmitter input. If the input signal is noisy at the transmitter, there is nothing else that can be done later to restore it to its original quality. The audio level is adjusted with the gain control on the transmitter, with some sort of level metering on either the transmitter or receiver. Transmitter gain set higher gives excellent signal to noise performance. The most difficult problem with properly adjusting the transmitter input gain involves duplicating the user's voice level in advance of the actual performance or use. Obviously, you need some sort of metering in order to correctly set the transmitter input gain. The metering must indicate the modulation level of the radio signal and also limiting in the transmitter. Page 9
Compression the Yellow Knob NOTE: we suggest you leave this set to the mid-position Compression may be adjusted between 1:1 and 5:1 Between the noise gate threshold and the rotation point (the point at which limiting takes over), the gain of the compressor should decrease with increasing input level. Modulation / Rotation Point (Limiter) - the Red Knob NOTE: we suggest you leave this set to the maximum position That is with the limiter threshold set very high you can then adjust this downward (increasing the limiter action) until the RED LED flickers occasionally. The limiter may be adjusted between 1:1 and 15:1 Above the rotation point (the point at which limiting takes over), the gain decreases at a higher rate with increasing input level, limiting the output signal. NOTE: if you set this control to low you will distort your audio signal. Internal Controls NOTE: Under normal operating conditions you are unlikely to need to change any of these settings, other than the Frequency selection switches. Audio Stereo to Mono Mixer The first audio stage is a Stereo to Mono (or dual channel Mono) Mixer, which provides a preset gain of 1 (link A) or 5 (link B), set by jumper PREAMP. The input impedance of each channel is 600 Ohms. High Pass Filter (Internally Adjustable) The next audio stage provides control over the high pass response for signals above 800Hz. This control will give an almost dead flat frequency response with the FIL "A" control in the mid-position. By default this control is set to midposition. Page 10
NOTE: The centre frequency is deliberately set lower than normal to cater for the needs of AM transmission. Whilst the audio stage circuits within the transmitter can work up to 25KHz, the actual bandwidth for any AM transmission system tails off between 5000-6000Hz (5-6KHz). Low Pass Filter (Internally Adjustable) The next audio stage provides control over the low pass response for signals below 800Hz. This control will give an almost dead flat frequency response with the FIL "B" control in the mid-position. By default this control is set to midposition. Compressor / Limiter A compressor is a device that squeezes the dynamic range of the incoming audio signal, reducing the difference between "loud" and "quiet". Basically, it has three sections as follows: Noise Gate (Internally Adjustable) For the very lowest signals, the gain reduces to reduce background noise. Used properly, a noise gate can help to eliminate background hiss completely, while letting the signal through (the hiss is still there, but you can't hear it when the signal is present). Used improperly, the initial parts of sounds are cut off, and the natural decay is not present. The control NOISE sets the threshold level for the noise gate, by default this is set to the mid-position Level Monitor The bar graph display (Green, Yellow and Red LED s) has three modes as follows: Audio Level Monitor (the default setting) In this mode the bar graph indicates the average level off the applied audio signal. This mode is selected when jumper MON has no link fitted (the default) Audio Level / Modulation Monitor In this mode the bar graph indicates the average level off the applied audio signal and average modulation level. This mode is selected when jumper MON has a link fitted, and the unit is operating Page 11
on the Internal ATU. This setting should only be used for initial set-up. Once set correctly revert to Audio Level mode (no link) Antenna Tuning Indicator (for use with External ATU setting only) In this mode the bar graph display indicates the level of mismatch between the transmitter and external ATU (Antenna Tuning Unit). This mode is selected when jumper MON has a link fitted, and the unit is operating with an External ATU. This setting should only be used for initial set-up. Once set correctly revert to Audio Level mode (no link) NOTE: When not needed the link can be stored on the two pins (lower left corner) marked Spare. Frequency Synthesizer The frequency synthesizer can be set to any frequency between 500 to 1800 KHz (US 10 KHz Channels) or 450 to 1620 KHz (European 9 KHz Channels). Antenna Tuning Unit Introduction Correct aerial tuning affects modulation linearity, and the maximum transmitter range. For optimum performance you should also set the antenna tuning capacitor. This a guide only and actual settings will vary depending on how the Aerial is positioned. ATU Operating Mode The transmitter has two distinct output modes: Internal The first uses the internal ATU to match an external antenna such as a loaded vertical. This comprises a separate tank and loading coil with trimmer capacitor. External The second has mode, is for use where the user wants to use their own external ATU, and comprises a tank coil only. Please Refer to APPENDIX F for details of how to set-up the ATU Page 12
Appendix A Block Diagram Page 13
Appendix B PCB Layout Appendix C - Convert Metres to Frequency The dials of the majority of British radios will be marked in wavelengths (metres) rather than frequencies. To convert from one to the other, divide 300,000 by the known figure (this conversion works both ways). The result should be rounded up or down to the closest whole number. Thus 300,000 divided by 1000 KHz gives 300 Metres. Conversely, 300,000 divided by 300 Metres gives 1000 KHz. Appendix D Ground Connections Run a heavy gauge ground wire straight down to a copper earth spike or water pipe and attach it with a brass grounding clamp (assuming a copper water pipe - always use compatible materials or corrosion will result) Page 14
Appendix E - Aerial Experiment (This information has been reproduced with kind permission of the Medium Wave Alliance ) For many applications, the simple 9 foot Ariel wire supplied with the Metzo will produce satisfactory results. Note: To Comply with FCC rules - be sure, that the total length of the Aerial, Antenna (Aerial) lead, and grounding wire is less than 10 feet as required by the FCC Part 15 rules. Since the actual wavelength at AM broadcast frequencies is over 950 feet, and the Metzo uses an aerial length of only nine feet, you may want to experiment with a loaded Antenna (Aerial) for better performance. An easy way to describe what happens when you load the Antenna (Aerial) is that you electrically increase the length of the Antenna (Aerial) without increasing the mechanical length. The goal is to approximate a functional portion of the full wavelength (i.e. 1/4 or 1/8 wavelength) Using an 9 foot length of wire, a series inductance of about 300-400 µh is required to properly load the circuit. Some Suggestions One way to make a simple and invisible aerial for use in a room is to wind 20 to 40 turns of thin insulated wire on 4 thumb tacks pressed into the back of a large picture frame, and then connect this to the aerial lead on the transmitter. Aerial Tuner and Loading Coil Most homebrew Antenna contain some reactance and have an impedance other than 50 ohms. A device called an Antenna tuner (also known as an "ATU") will transform your Antenna system into a non-reactive load of the proper impedance. A properly matched Antenna (Aerial) will radiate more efficiently and, in some cases, will produce a better-sounding AM signal. Loading Coils Instead of a complete and complex Antenna tuner, many LPAM stations use a variable inductance located at the transmitter output jack or at the feed point of the Antenna (Aerial). This approach is popular with Part 15 experimenters using simple wire Antenna (Aerial)s or shortened verticals. First you insert a tiny "grain of wheat" light bulb, a flashlight bulb, or a similar small light bulb between the inductor and the Antenna (Aerial). (Stations using 100 mill watts might need a bulb rated for 1.5 or 3 volts at 20 to 60ma) Then you try various inductances while the transmitter is on, and see which setting makes the bulb glow most brightly. Then remove the light bulb from the circuit and transmit. The variable inductor can be, for example, a coil made of 100 turns of 22-guage (#22 AWG) enamelled wire that you wind around a piece of PVC pipe 2 or 3 inches in diameter, with taps (connection-points) built into the coil every 10 turns; or any similar inductor. The various taps in your coil can be wired to a rotary switch, or you can use a small alligator clip to select one at a time. The diagram to the left shows a more elaborate scheme that can be used with a vertical Antenna (Aerial). One end of the coil is grounded, in other words, connected to earth-ground. That lower third of the coil is made with several taps 3 or 4 turns apart, and the transmitter can be connected to any of these taps on the ground end of the coil. From the middle to the top end of the coil are several taps about 10 turns apart, and the Antenna (Aerial) can be connected to any of these upper taps. For this application, try the following coil dimensions: 145 turns of #20 AWG enamelled wire, close wound (no spacing between turns), on PVC pipe having an outside diameter of 3.5 inches. The total inductance of this coil is about 1 milli-henry, enough to base-load an 8-foot whip Antenna (Aerial) at frequencies Page 15
of 1000 khz and above. Use a field strength meter or the light bulb trick to find the best taps for both the Antenna (Aerial) and the transmitter. This system of tapped inductors only provides crude tuning and is not likely to take you to the point of exact resonance. If you are building your coil on PVC pipe, use white PVC pipe, not grey electrical conduit material or black sewage pipe material. These different types of plastic have different dielectric properties and the white stuff is best for radio work. After building the coil, you can apply a coat of clear lacquer to help hold the wire in place. And yes, all that wire in the loading coil does introduce some resistive losses and a significant amount of RF power is lost along the way. However, the improved radiation efficiency of a properly matched Antenna (Aerial) more than makes up for it! Delivering one milli-watt of power to a properly matched Antenna (Aerial) will produce a much stronger signal than delivering 10 milli-watts to a severely mismatched Antenna (Aerial). APPENDIX F ATU SETTINGS The picture to the right shows the various strapping arrangements for the ATU. The first picture shows the arrangement when using an External ATU (the transmitters own ATU is out of circuit) The next three pictures show the strapping for the ATU for various frequency bands from 500 KHz to 1700 and above KHz. FAQ Audio distorted Hum on audio No Audio Audio source volume to high. Limiter (Rotation Point) is set to low (the RED LED is ON!!!) ATU not set correctly. Not using the GROUND connection on the transmitter. Bad GROUND connection. Bad audio cables - check for broken shield wires. Power supply filtering poor, (not using a regulated power supply). Bad audio cables. Audio source volume set to minimum. Page 16
Specifications Frequency Range 500 to 1800 KHz in 10 KHz steps (US Mode) 450 to 1650 KHz in 9 KHz steps (UK Mode) Crystal Frequency 10.240 MHz (US Mode) for 10 KHz channels 4.608 MHz (EU Mode) for 9 KHz channels Frequency Selection 8 way DIP switch Frequency Adjustment ± 0.0005 (via trimmer) (Optional Extra) Frequency Stability @ 25?C 0.003% Modulation Type Amplitude Modulation (AM) Maximum Modulation Level 100% Power Input to RF Amplifier Antenna Matching Tuning Marker Tone 100mw (US Mode) to comply to FCC Part 15 rules Antenna Supplied with 3 metre wire Antenna (Aerial) 100mw (UK Mode) Antenna Socket RCA Socket (universal 50 75Ω) Inbuilt ATU, with trimmer to peak Antenna resonance. RF Output Monitor Inbuilt LED bar graph RF power monitor (for External ATU only) External ATU RF Ground Separate transmitter ground terminal Inbuilt tone generator for transmitter set-up and Antenna tuning Audio Response 10 Hz to 25 KHz Audio Stage 625 Hz 30 Hz to 6 KHz RF Stage 1250 Hz Filters Treble Boost / Cut Audio Distortion 0.5 % THD (Audio Stages) Bass Boost / Cut Audio Input Sensitivity 200 mv RMS for 100% modulation Audio Limiting Adjustable 151 compression above threshold Audio Compression Adjustable from 1:1 to 5:1 Audio Noise Gate Audio Limiting Threshold Adjustable limiting threshold Front Panel Controls Audio Gain Front Panel Indicators Rear Panel Connections RF / Audio Monitor (3 bar LED) RF RCA, Adjustable noise reduction threshold Compression Limiting Audio Channel 1 & 2 RCA Power Consumption 15 VDC @ 300 ma DC Power 2.1mm Dimensions Width 140 mm Depth 110 mm Height 35 mm Ground M3 Simple Warranty We guarantee the product to be free from defects in materials and workmanship for one year. We will repair or replace such product, if found to be defective in materials or workmanship. All returns must be accompanied by a written statement setting forth the name, address and daytime telephone number of the owner, and email address together with a brief description of any claimed defects. The customer shall be responsible for all costs of transportation and insurance, in both directions. We will make all reasonable effort to repair or replace the unit covered by this warranty within fourteen days of receipt. This warranty shall be void and of no force of effect in the event a covered product has been modified in design or function, or subjected to abuse, misuse, mishandling or unauthorized repair. We hope your Metzo gives you may hours of enjoyment! Page 17