2M VHF FM Receiver
Neohd
Posts: 5
Usually I never ask and always attempt to find my own information on the site or the web in general. I have come to a cross road and can not find any further details around what I seek. I have found some very basic explainations and it has helped alot, but I am stuck on a few things and hopefully someone can shed some light on this subject for me, or at least point me in the correct direction.
I am a HAM radio operator and very new to RF design and such. What my goal to do is to create a 2M VHF receiver that is fully controlled by a Propeller. I do not know radio design that much so I am a little bit limited, but I am very much eager to learn if pointed in the correct direction. I would like to create a receiver that will be able to receive the HAM band and a little bit outside of this using the PLL. Is this possible using the Propeller? Is the PLL that is built in to the Propeller capable of providing the control that I desire that other standalone PLL chips can provide, but still yet remain fully customizable?
What I have learned so far in FM radio receivers is very basic but interesting. I have realized that most FM receivers use an Intermediate Frequency(IF) to make the design of the IF Amplifier, FM demodulator and Audio Amplifier sections basic and are tuned specifally to the IF and make the design simpler. The part that I am confused on is how to control the selected or listening to frequency using the Propeller PLL or similar type of method.
I understand that my understanding is basic, but hopefully someone or the forum as a whole can help point me in the correct direction.
Any help is greatly appreciated and thank you in advance.
-73
-Neohd
I am a HAM radio operator and very new to RF design and such. What my goal to do is to create a 2M VHF receiver that is fully controlled by a Propeller. I do not know radio design that much so I am a little bit limited, but I am very much eager to learn if pointed in the correct direction. I would like to create a receiver that will be able to receive the HAM band and a little bit outside of this using the PLL. Is this possible using the Propeller? Is the PLL that is built in to the Propeller capable of providing the control that I desire that other standalone PLL chips can provide, but still yet remain fully customizable?
What I have learned so far in FM radio receivers is very basic but interesting. I have realized that most FM receivers use an Intermediate Frequency(IF) to make the design of the IF Amplifier, FM demodulator and Audio Amplifier sections basic and are tuned specifally to the IF and make the design simpler. The part that I am confused on is how to control the selected or listening to frequency using the Propeller PLL or similar type of method.
I understand that my understanding is basic, but hopefully someone or the forum as a whole can help point me in the correct direction.
Any help is greatly appreciated and thank you in advance.
-73
-Neohd
Comments
Some suggestions that could get you started:
Research FM modulation so that you understand IF to AF conversion
Research mixing
Get the book "Basic Communications Electronics" ISBN 0-945053-24-X, it has a lot of really useful information in it.
There is another problem you are going to run into, the counter speed. I know the data says 500Khz to 128Mhz, but all I've seen is that the counters run at main clock speed and can't run faster than main clock speed. You are dealing with a range of 144-148Mhz, so you can't directly generate that frequency. You will need an external oscillator for your primary detector, but you can generate the If and probably do the AF conversion in software.
See PLL Modes of Operation in AN001, but in brief it works like this:
You configure FRQA to generate a frequency between 4 and 8MHz ( 4MHz =< FRQA * CLKFREQ / 2^32 =< 8MHz ). This is then multiplied by 16, then divided using PLLDIV.
Fout = Fnco * 2 ^ (PLLDIV - 3)
Fnco = FRQA * CLKFREQ / 2^32
I read that, but it wasn't clear whether it was the CTR or the main clock that was being used. It seemed to indicate the main clock oscillator was used for the PLL. I just felt the PLL section of the AN was a little obtuse (for that matter, the whole AN is a little brief and glosses over some really important stuff, like the theory of how each of the functions works). Also, it was a little unclear that the PLL applies only to the output pin and not to the timescale of the counter. Most other micros allow you to pick the clock source for the counter and coming from that paradigm, I had initially assumed the PLLDIV divided the CLKFREG * 16 down to the desired master clock frequency, then the accumulators worked as normal. It seems to me that would have actually been the better way to do it, then you can use the CTRs for timebases natively or getting a certain resolution for external events.
Once I understand something completely, I'm really good and explaining it, but the counters are a little fuzzy to me still.
Maybe I'll go at that AN again and just rewrite it to give complete understanding! :nerd:
I once tried generating RF at 146 MHz using just a counter in PLL mode. I was able to detect very strong birdies all up and down the band (and outside) due to the PLL's jitter. 'Definitely not recommended.
For something simple to try, you could replace the crystal in Parallax's FM radio module with one of a higher frequency and see if it still works to pick up two-meter transmissions.
-Phil (AD7YF)
Yes.
No.
You need a much better spec'd PLL for RF reception, than any Microcontroller built-in PLL, but the Prop can certainly control an external PLL Synthesis chip.
You can easily work out the jitter effects, especially on FM.
If you take a 150MHz ballpark, and add 1ps of jitter, that is ~22KHz of frequency change.
Look at Analog Devices or Silabs ? and I think someone mentioned the Si570 has been used for this sort of work...
For best purity, you would make your own VCO.
-Phil
I assume you are referencing the note in the AN001 that says any FRQx value that isn't a power of 2 will create jitter? That's what we are talking about?
-Phil
Phil has another thread (link?) where Phil describes the demodulation process very clearly using the prop.
The SDR does not use an IF, but instead demodulates down to values around 96KHz??? (the bandwidth of a pcs audio card) and the pc is left to filter out the audio. To me, the prop should be able to do both tasks from the work Phil has done, because the prop has multiple cores.
Ray (VK2ZTZ)
-Phil
I haven't got any PC cards that go above 48KHz but I have dreamt about SDR. I have scrounged a bunch of old analog TV tuners to play with the I2C programming but the finest freq step with these is 50KHz, but I have an idea about chaining two of them with one on 50KHz steps and one on 62.5KHz steps to try for 2.5KHz difference (this would probably end up as the worlds best noise generator!).
I have a 74HCT4046 here that I mean to try as Phil suggested as an external "jitter reducing flywheel" . I have tried to produce a 6.975MHz (I think) for a teletext problem and built a LC filter for it. I stuck it onto a spectrum analyzer, at work, and the jitter although reduced was still there as noise sidebands. Whilst it wouldn't have made any problems for the teletext I am sure it would have screwed up 2.5KHz deviation demod.
Alan (ex G8LCU)
http://groups.yahoo.com/group/softrock40/
They use simple I/Q mixers with sound cards and DSP on the PC, and some members have been using them for 2m reception with converters:
http://groups.yahoo.com/group/softrock40/message/26580
I read that using a DDS for fine tuning of the frequency can be used especially since it will change its frequency faster. I could be wrong. Again still a learning thing. Does one have to use 10.7Mhz as the final IF?
See my possible idea and let me know if you see something that will not work or just plain does not make any sense?
73
-Neohd
As far as the speed of fine tuning, a DDS will give you nearly instant QSY but, for small frequency changes, a PLL would probably do just as well.
One thing I'm not sure about is how low a final IF you can use for FM, since you need some degree of bandwidth just to accommodate the modulation. IIRC, amateur FM is narrow-band, but I don't remember what the bandwidth is.
-Phil
Addendum: 'Just checked the spec sheet for my VX-2R HT. For NFM, the final IF is 450 KHz; for WFM, it's 1MHz.
Thank you for the response. What should I do to filter out the first IF? Is the band pass filter not good enough or is it two wide? As far as narrow-band for HAM Radio operators, it is not widely used, amateurs in the US still operate primarily on wide band. The narrow-band 2013 FCC mandate that has come out is only for Public Safety and Industrial/Business use of the spectrum only. The Amateur Community maintains the spectrum they are allocated by the FCC, which you might already know. The bandwidth size for Wide and Narrow bands are 25kHz and 12.5kHz respectfully but this varies from Type 90 and Type 97(amateur) equipment, I don't know why probably stricter rules. I believe that broadcast WFM is about 200kHz in width. This is going from memory so hopefully this is correct. If these numbers are correct, then I believe that 455kHz is wide enough even for the wide band which is what I will primarily be listening to. The diagram I listed above can also be used for more than just the HAM bands in the 2M range.
Do you have any suggestions that will improve upon this design, aside from what you might have already stated? I would like to create a further stage to filter when selecting Wide vs Narrow, but this is just a wish and might go further than I currently understand. I will improve upon the final IF and make it higher. Any suggestions on the FM demodulation? I did see someone mention that you think the Prop can perform this function? Or is this also better left off loaded to another specialty IC?
73
-Neohd
Correction: I believe that I found the FM Demodulation I was asking about.
The Propeller should be able to do any baseband filtering that's necessary on the I and Q demodulation products. See this thread:
Scan down to post #6, once you've read the introductory material.
-Phil
if the whole 144-148 band can be received if its mixed down to that range
such as with a SA612 mixer being driven with a cut (modified) 16Mhz crystal
in overtone mode. (16 x 9 overtone is 144, but re-cut it to be 140 or 152 ?).
edit:I once did something just like that to listen to 6 meters on a 10 meter rig.
On the forum a year or two ago, someone made an I+Q AM broadcast receiver in pure code,
which worked well with an antenna wire connected to a resistor on two prop pins.
But,
I don't have an idea at the moment for demodulating the FM after transverting the whole band to 4Mhz.
Just remember, you have 8 cores running at 80MHz (or overclock depending of the pcb layout to 104MHz). Phil has done a fantastic job of explaining the I&Q demodulation including the maths.
IMHO this is certainly worth a try with the prop.
Phil,
I do have a question. I have been trying to read up on the Image Frequency and calculate it so I can properly filter it before the First Mixer stage. What I have found so far is Fi = 2Flo + Fc and Fi = 2Flo - Fc. Given the range that I plan on receiving in initially which is, 144Mhz - 148Mhz, will image frequency be a problem? From what I can gather it is a problem if the image frequency is very close to the First IF, is my understanding correct? Eventually I would like to expand on this design and broaden its frequency range if possible, but for now this is just my inital understanding.
If my first IF is 10.7Mhz and my intended receive frequency is 21.4Mhz, then this particular frequency would be a problem with my receiver, is this understanding correct? If this is not correct, how does one properly filter prior to the first Mixer and prevent Image frequencies from interfering on such a broad range of frequencies?
73
-Neohd
From the second,
That latter value is an image frequency and happens to be 2 * 133.3 - 144. At the high end of the band, the image frequency will be
So the range of image frequencies that must be filtered out ahead of the mixer is 122.6 - 126.6 MHz. An LC bandpass filter constructed to pass 144 - 148 MHz should work, as would a tunable filter with a higher Q (i.e. a preselector).
It should be obvious from this that the lower the IF, compared to the receive frequency range, the harder it will be to reject image frequencies, since they will be so close to the desired reception band.
-Phil
Addendum: The image frequency range, 122.6-126.6 MHz is in the aircraft band, where AM predominates. So, barring front-end overload, your FM detector should also provide some "image rejection" just on that basis.