{"id":5744,"date":"2013-01-12T20:50:42","date_gmt":"2013-01-13T04:50:42","guid":{"rendered":"http:\/\/rustybolt.info\/?p=5744"},"modified":"2013-12-01T09:36:04","modified_gmt":"2013-12-01T17:36:04","slug":"2012-01-12-fm-microphone-everything-you-wanted-to-know-and-then-some","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=5744","title":{"rendered":"2012-01-12 FM Microphone- Everything You Wanted To Know And Then Some"},"content":{"rendered":"

I built up this FM microphone from belza.cz<\/a>.\u00a0 The circuit is super simple, uses only a single transistor.\u00a0 The output of the electret condenser microphone is coupled directly to the base of the RF oscillator, and causes the frequency modulation.\u00a0 However since the microphone has no amplification, it is not sensitive and the user must speak close to the microphone.<\/p>\n

Earlier FM Mics I’ve Built<\/h3>\n

I’ve built several of the two transistor FM microphones that are often seen on the ‘Net.\u00a0 They typically use a RF stage that is different than the one seen at Belza.cz.\u00a0 These typically have an NPN transistor, often a 2N3904.\u00a0 The base is at RF ground, typically by connecting a 1000 pF capacitor from the base to ground.\u00a0 There are two (or sometimes one) base bias resistors, typically 10k and 6.8k.\u00a0 The emitter is connected to ground through a 470 ohm resistor.\u00a0 The collector has the parallel resonant tank connecting it to the positive supply.\u00a0 This parallel resonant tank is typically a 120 nanohenry (same as 0.12 uH) coil and a 20 pF capacitor connected in parallel.\u00a0 Along with the transistor’s capacitance, it resonates at the low end of the FM broadcast band [see Note].\u00a0 The feedback capacitor, typically 4.7 pF, is connected between collector and emitter, and causes the circuit to oscillate.\u00a0 This stage is preceded by a single transistor to amplify the microphone.\u00a0 This amplified audio is applied to the base where it changes the transistor’s capacitance and that causes the frequency modulation.\u00a0 This type of RF stage forms a fairly reliable transmitter.<\/p>\n

But the FM mic at Belza.cz is quite different.\u00a0 It uses a PNP 2N3906.\u00a0 Again, the base is at RF ground due to C1.\u00a0 The collector has the coil, but the capacitor(s) for the coil are C2 and C3, connected in series<\/span> and the center connection is also connected to the emitter.\u00a0 I’ve built similar RF stages before and been disappointed because they sometimes did not oscillate or they weren’t stable.\u00a0 I decided I would stick with the more common type without the series capacitors.\u00a0 But I’ve built several other circuits from Belza, and they all lived up to expectations.\u00a0 So I decided to try this one for a change.<\/p>\n

I made a few changes to Belza’s circuit.\u00a0 I used a 470 pF and 330 pF in parallel for C1 – it’s an RF bypass cap and is not critical.<\/p>\n

To get the right voltage, I used 2.7k for R1 (the 4.7k was too high).\u00a0 R2 actually measured 360 ohms.\u00a0 I used a 20 pF NPO (NPO means zero temp coefficient) for C2, less than shown because the other circuits I mentioned use only about 5 pF.\u00a0 For C3 I used a 36 pF, but when I found that the frequency was too high, I paralleled a 5 pF to it for a total of 43 pF.<\/p>\n

The All Important COIL<\/h3>\n

The coil of the FM microphone is important because it along with the capacitor(s) determines the frequency and stability.\u00a0 Also it is usually small, in order to keep\u00a0 the FM mic small.\u00a0 In order to tune the FM mic, the capacitor can be a variable capacitor, but they are usually much bigger than the fixed ceramic disk and are more expensive.\u00a0 So many inexpensive FM mics use a fixed capacitor and a coil that can be squeezed or compressed to tune the frequency.\u00a0 Even if the coil is wound on a coil form with a small tuning slug, it is smaller and may be cheaper than the variable capacitor.\u00a0 But whatever the coil is, it should have good mechanical stability; the copper wire should be thick enough to support the coil firmly, and the coil should be small and light so it doesn’t pick up vibrations.\u00a0 Vibrations, also called microphonics, can be heard in the FM signal when it’s quiet.\u00a0 One way to muffle this is to stuff the coil with a tiny piece of foam rubber and when the coil has been tuned, drop a few drops of paraffin AKA candle wax on it.\u00a0 The experimenter can use a short piece of soda straw as a coil form, and some hot glue, silicone seal or other glue to hold the coil in place.<\/p>\n

One other method of making a coil is to use a spiral shaped trace on a PC board.\u00a0 This is very stable but is not variable, however it can have multiple taps to give some range of tuning.\u00a0 We’ll just leave it at that, because it has to be done when the board is made and few experimenters want to make their own boards.<\/p>\n

However, I’ve made the equivalent to that by using a short length of 14 AWG or other heavy gauge copper wire, and I bent it into a square shaped single turn coil.\u00a0 Each side of the coil was about 35 to 40 mm or 1-3\/8 to 1-1\/2 inches on a side.\u00a0 This was held up above the circuit board by putting blobs of silicone glue on the board, letting them dry, then mounting the coil on the silicone blobs with more silicone glue.<\/p>\n

My coil was at first about 120 nanohenrys, which is typical for these FM mics.\u00a0 It was 22 AWG solid enameled wire, and about 5 turns, inside diameter of 5 mm or 0.196 inch.\u00a0 The frequency was up around 94 MHz, too high – I wanted it around 90- MHz.\u00a0 So I removed the coil and put in one made of 26 AWG solid enameled wire and was 10 turns close wound on a much smaller 0.120 inch diameter air core.\u00a0 The turns are touching each other so it is quite stable, but then it can’t be squeezed to tune it.\u00a0 But the inductance is about 200 nanohenrys and the frequency is about 89.8 MHz.<\/p>\n

Not shown on the schematic but is absolutely necessary at this frequency is a 0.01 uF ceramic disk bypass capacitor across the power supply lines.\u00a0 I usually run these FM mics with no antenna because the antenna is connected to the oscillator and any changes to the antenna change the frequency.\u00a0 Without the antenna the FM mic may be picked up 300 feet or 100 meters away, but the distance is heavily influenced by the battery voltage, which determines the power.\u00a0 This one uses only 3VDC and draws about 3\u00a0 mA, which totals only 9 mW, and consequently the signal can’t be heard more than a few dozen feet.\u00a0 I could add the antenna but I don’t really need the greater range.<\/p>\n

Microphone Sensitivity<\/h3>\n

I was surprised at the sensitivity of the microphone.\u00a0 It’s not sensitive, but it’s not insensitive.\u00a0 I can put my “ticking time bomb” circuit a foot away and clearly hear the ticking in the FM radio a few yards away.\u00a0 I have to talk about a foot or two away, whereas the circuits with the second transistor as microphone amp can pick up sounds a dozen feet away,\u00a0 So this single transistor circuit can be sensitive enough for use as a close microphone.\u00a0 One other point that I’ve run into.\u00a0 The electret condenser microphone is a strange beast.\u00a0 Its range of sensitivities can be wide, as can its current.\u00a0 The circuit called for a 4.7k for R1, but I had to use a 2.7k to get enough voltage across this resistor, which determines the transistor’s operating point.\u00a0 So each microphone will have to have its resistor selected to match it.\u00a0 In other words, don’t expect the circuit to work good if you put a 4.7k in there and forget to check the voltages.\u00a0 Like it says in the schematic, there should be about 1.3 volts across the resistor.<\/p>\n

Another problem I’ve found with the microphone is its low frequency sensitivity.\u00a0 The human ear doesn’t respond to low frequency sounds, such as when a door closes and causes the pressure in the room to change.\u00a0 This circuit has the microphone connected directly to the transistor, so its low frequency response is very good, probably too good.\u00a0 When some low frequency sound is picked up, it will cause the FM signal to vary widely and this could be a problem.\u00a0 The circuits that use a microphone amplifier can be limited in their low frequency response by using lower value coupling capacitors, so the frequency response drops below 100 Hz, where there are little or no human voice input.\u00a0 This circuit doesn’t have that option – you get maximum bass response from the microphone.<\/p>\n

Frequency – Changes<\/h3>\n

One other point that might be a concern.\u00a0 As more receivers today are all digital, and they have channels that are on the FM frequencies such as 88.1, 88.3, 88.5, etc., it may become harder to pick up one of these FM microphones if it is on a frequency that is not a channel, such as 88.2.\u00a0 And if it is close enough to a channel it may be picked up but as the battery voltage and\/or temperature changes, it may drift to a frequency that can’t be received.\u00a0 So it might be a good idea to use a coil that can be spread or compressed to adjust the frequency somewhat.<\/p>\n

Another way to tune an oscillator is to put a diode across the capacitor, and connect it to a potentiometer so that a variable DC bias voltage can be put across the diode.\u00a0 But this usually requires more than 3 volts across the diode to give enough tuning range.\u00a0 If the diode has from 3V minimum to 9V or so, it will give a reasonable range of tuning.\u00a0 One way to get 9V for the diode is to use a pair of 3V lithium coin cells.\u00a0 This when added to the 3V supply will give 9VDC.\u00a0 The diode doesn’t draw any current, but the pot does.\u00a0 But if the pot is 1 Meg, the current drawn is so low that the coin cells should last for years.\u00a0 Another cheap way to tune it slightly is to vary the base bias.\u00a0 But this also changes the power output.<\/p>\n

Note:\u00a0Some information on how to determine the capacitance and inductance for this type of microphone.\u00a0 Let’s say you want to use a coil with 120 nanohenrys or 0.12 microhenrys.\u00a0 You can plug this into a calculation on a scientific calculator, and you will come up with the reactance at 90 MHz, the frequency I’ve chosen.\"IMG_20131201_090327\"<\/a><\/p>\n

 <\/p>\n

 <\/p>\n

 <\/p>\n

I multiply 2 times Pi times the frequency 90E6 Hz or 90 Megahertz, times 120E-9 or 120 nanohenrys.\u00a0 The result givces about 68 ohms inductive reactance.<\/p>\n

The capacitor has to have the same reactance at the resonant frequency, so we plug this into a calculation to get the capacitance.\"IMG_20131201_090214\"<\/a><\/p>\n

 <\/p>\n

 <\/p>\n

I divide 1 by 2, by Pi, by the frequency 90E6 or 90 Megahertz, and by the reactance, 68 ohms.\u00a0 The answer 2.6 E-11 has to have the decimal point moved one to the right, to give 26 E-12 or 26 picofarads.\u00a0 This is the total capacitance across the coil.\u00a0 There is a small ampunt of capacitance in the transistor’s collector, about 4 or 5 pF for the 2N3904, so the actual capacitor value should be about 20 to 22 pF.\u00a0 If the coil can be stretched or compressed, it should give a range of tuning to make the frequency somewhere around 90 MHz at the bottom of the FM band.<\/p>\n

 <\/p>\n

 <\/p>\n

Back to experimenting…<\/p>\n","protected":false},"excerpt":{"rendered":"

I built up this FM microphone from belza.cz.\u00a0 The circuit is super simple, uses only a single transistor.\u00a0 The output of the electret condenser microphone is coupled directly to the base of the RF oscillator, and causes the frequency modulation.\u00a0 However since the microphone has no amplification, it is not sensitive and the user must <\/p>\n

(Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8,7],"tags":[],"class_list":["post-5744","post","type-post","status-publish","format-standard","hentry","category-audio","category-radio-frequency"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5744","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=5744"}],"version-history":[{"count":28,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5744\/revisions"}],"predecessor-version":[{"id":5746,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/5744\/revisions\/5746"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5744"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5744"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5744"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}