FMRANGE www.hopco.com/fmradio Copyright 1997 HOPCO INTRODUCTION To most users, the PC-compatible FM Radio Card that is so popular these days is a convenient and simple to use accessory. Few know how it works, nor do they care. But, to some curious minds, there is always a need to know more. We at HOPCO are of that sort, and so we invested considerable effort in delving deeper into the innards of the card. We came out having a rather complete understanding of its design, operation, and capabilities. Here we present some basics and specifically discuss the factors that limit tuning range. The FM Radio Card is a rather conventional stereo FM radio tuner except for its novel PC interface, control software, and audio output technique. The radio design utilizes a single chip tuner employing single-conversion technology. Digital tuning is accomplished by a Phase Lock Loop (PLL) chip. On-board volume control permits setting four levels, one of which is OFF. To achieve better audio level control, the output of the FM Radio Card is normally fed to the line-input of a PC sound card. This, in turn, can be controlled (by software) to a finer degree. If you feed the output to a stereo amplifier, you can use the amplifier's volume control, of course. But the on-board capability limits you to only one of four volume levels. FACTORS AFFECTING TUNING RANGE A number of factors affect tuning range of this or any radio: * Inherent limitations of the radio chip employed, * Capability of the PLL frequency synthesizer, * Choice of critical tuning component values, * Limitations of the controlling software. All these factors come into play with this radio. The radio chip used actually is capable of handling both AM and FM reception, though only components for FM are installed. Presumably, this choice was made both to save on cost and because the noise generated by a PC in the AM broadcast band would be horrendous. FM RADIO DESIGN BASICS A few words about FM radio design are necessary before we proceed. A single-conversion design works as follows: The broadcast FM signal from the antenna is passed through a tuned network to filter extraneous signals (actually, to just attenuate them somewhat). This signal is then "mixed" with a local oscillator (LO) signal which, in this case, comes from the PLL. This signal is offset from the frequency of the station being received by an amount equal to the Intermediate Frequency (IF), which is the common 10.7 MHz. The IF signal is filtered and amplified further and then passed through a "quadrature detector" which recovers the audio information from the FM signal. This audio is amplified, level set, buffered and fed to the output connector. In this (as in most) design, the PLL LO-signal uses "high-side injection," which means it is always set to 10.7 MHz ABOVE the frequency of the station being received. Note that "low-side injection" could also be used, in which case it would be set 10.7 MHz BELOW the received frequency. You can demonstrate this by picking a strong local FM station above 100 MHz. To force the PLL to the proper frequency for low-side injection, use your normal FMRADIO program and set it to a frequency that is 21.4 MHz lower than the frequency of your chosen station. For example, if your station is at 104.1 MHz, tell the program to tune to 82.7 MHz (104.1-21.4). Voila! There's your station, loud and clear (well, sort of anyway). You must ensure that the frequency you pick is high enough that the FM Radio Card can actually tune to the low side of it. Since most cards will tune at least to 79 MHz, the above example works. What's going on here? Well, recall that your control software thinks it's using high-side injection. So, it adds 10.7 MHz to whatever frequency you specify for reception. In this example, you entered 82.7, so it actually set the LO to 93.4 (82.7+10.7). But this is a LO signal exactly 10.7 MHz BELOW 104.1 MHz, and so the DIFFERENCE between them still comes out 10.7 MHz and you hear your station! FM Magic! The astute among you will observe that we will also hear any station that happens to be on a frequency of 82.7 MHz -- after all, that's what we set the radio to. You're right! Two frequencies are received at the same time: one 10.7 MHz (or whatever the IF frequency is) ABOVE the LO frequency and another 10.7 MHz BELOW it. This is where "front-end filtering" comes into play. Only the rejection characteristics of the filter placed ahead of the mixer can reject (attenuate) one of these stations and enhance the other. This (and most) FM radio designs use simple filtering, counting more on a judicious choice of frequencies to avoid interference. But if you are going to be experimenting and modifying your FM Radio Card to cover other frequencies, you should be aware of this "image frequency" reception. It's present in all radios! RF FILTERING The "front end" filter in your FM Radio Card is electronically tuned as you change the PLL frequency. It consists of simply an inductor and an electronically variable capacitor. The inductor is one of two coils of about five turns of copper wire you see oriented vertically closest to the mounting bracket. If you wish to lower the reception range of your radio, you must increase this inductance. On the other hand, increasing the reception range requires lowering the inductance. We'll discuss later ways of achieving this. Another thing which can affect sensitivity at various frequencies is the frequency range of the RF amplifier and mixer in the radio chip. While we were unable to find a published specification from the manufacturer on this limit, we would estimate that FM mode is limited to a high of about 200MHz or less. In experiments we were able to push it to 150MHz, though sensitivity did suffered somewhat. PHASE LOCK LOOP While the FM radio chip handles all the radio reception tasks, it's the PLL which I consider to be the heart of the radio. This chip accepts frequency commands from the PC and determines what station you listen to. It is a very complex and powerful chip, but the end result is that it generates a LO signal of the correct frequency. As we said before, due to "high-side injection," this frequency is always 10.7 MHz greater than the frequency you tell it to receive. Tuning is actually in steps of 25 kHz (0.025 MHz). This is a finer step size than used by normal FM Broadcast stations. With proper software (such as our FMSCAN.EXE) you can tune between stations. Also, the tuning range of the OEM supplied FM Radio Card software (87.5 to 108 MHz) is an artificial software limitation. We tested a number of cards and found that, in all cases, the hardware was capable of a range of at least 79-114 MHz without modification. What determines the tuning range is one inductor and one electronically variable capacitor, just like the front-end tuning. In this case it is the other coil of copper wire: the one oriented horizontally and second in line away from the mounting bracket. You can alter the tuning range of your radio card by changing the value of this inductor. However, the PLL chip specifies a limit of 45-130 MHz, though this might be pushed somewhat. MONITORING TUNING VOLTAGE There are a pair of gold-plated pins protruding from the card along its top edge 1.5" behind the mounting bracket. These pins are connected together and permit monitoring the PLL (and front-end filter) tuning voltage. Simple place a high-impedance DC Voltmeter between either of these pins and ground (e.g., the bracket itself). As you change the frequency, you will observe a change in voltage: from zero at the lowest achievable frequency to around 12-13v at the highest range. We tested a number of radio cards with the following average results. FREQ Tuning Voltage 77.5 0.10 88.0 1.00 98.0 2.80 108.0 6.70 116.0 13.15 SUPPLY VOLTAGE A word should be said about supply voltage. The FM Radio Card uses both the +5 and +12 volt supplies. Specifically, the electronically tuned capacitors (both for front-end filtering and PLL frequency) utilize the +12v source. Frequency is directly proportional tuning voltage. So, the greater the value of your +12v supply, the greater the high-end tuning range. You will note in the above table that our particular supply went to +13.15 volts. If yours is lesser or greater, it will affect the highest frequency you may reach. We have not as yet explored the possibility of using a higher supply voltage (for tuning only) to achieve a wider range. Instead, modifying the inductor values achieves this end more simply and directly. CHANGING INDUCTORS With our FMSCAN program you are able to enter a wider range of frequencies than are possible with the OEM supplied FM radio software. However, you without inductor modification, you will be limited to the tuning range of the inductor/capacitor installed on your card (about 79-114 MHz). To cover a wider range you will need to change either the inductor or the capacitor (or both). Since the capacitor is actually an electronically tuned "varicap," it is simpler to leave it alone and modify the inductor(s). While changing just the PLL inductor will alter the tuning range, you should modify the front-end filter inductor to achieve maximum sensitivity at the new frequency. Generally, it is suitable to make the same modification to both, as the front-end filter is not very sharply tuned. The simplest way of altering these inductors is to remove the silicone caulking that holds them in place and simply squeeze together or spread apart the turns of the coils. You may do this with your fingers, but if you wish to make the change under power, a plastic (non-metallic) tool of some sort will keep the presence of the tool from affecting the inductance. Always make a final check with the card mounted in place and no tools or fingers or cables near those coils. A useful tuning technique is to monitor the tuning voltage on the test pins as described above while making changes to the inductor(s). Ensure that this voltage stays between zero and the level of your +12v source over the range of tuning you desire. If you wish to set the inductors back to their original value, simply set a frequency of 99 MHz and adjust the PLL coil for 3.0 VDC on the meter. Then set the front-end coil so it looks similar. Obviously, you could also remove the inductors and replace them with new ones you wind yourself. This might be indicated if you cannot achieve the desired tuning range by simply squeezing and spreading. However, there's another alternative: ferrites. Any ferro-magnetic material placed inside (or near) the coils will alter the inductance considerably. Ferrite is one of the best materials for this purpose, as it is available in special formulations for operation at these frequencies. In one experiment, we broke a small ferrite toroid in thirds and glued a piece inside each coil. Use RTV (bathtub or window caulk) for this purpose. We were then able to tune the low VHF TV band (Channels 2-6) and listen to TV station audio on the FM Radio Card. The audio FM signal is just below the upper limit of each channel (CH2:54-60, CH3:60-66, CH4:66-72, CH5:76-82, CH6:82-88). We used high-side injection in this experiment, and obviously the FM band itself represented an interfering "image frequency" source on the upper three channels. Nevertheless, strong local Channel 3 TV came through loud and clear without even any changes to the front-end filter coil! CONCLUSIONS We've presented here various factors that affect the tuning range and capability of your FM Radio Card. And we've presented means of altering the frequency and determining that all is working properly. Our FMSCAN.EXE program permits you to both automatically scan and manually set frequencies well outside the normal FM broadcast band. That program, combined with the information presented here will permit you to experiment with other applications for your FM Radio Card. If you come up with something interesting, or just collect useful test data, we'd appreciate hearing about it. Please EMAIL us at fmradio@hopco.com anytime.