Help testing my FM receiver - Page 1
Forum section: electronics / beginners

author: helix1
author_rank: Contributor
timestamp: January 29, 2016, 10:39:36 pm
content: I'm trying to build the FM receiver as shown in the first circuit below. The audio part is working fine but the RF part of the circuit is not giving any output at all right now. I've constructed the circuit on perf board, and have done basic checks to make sure there are no wiring errors so I now need to look deeper to find what's not working.The second figure below shows a more detailed look at the TDA7000 chip. My strategy for debugging this is to try following the path of the RF signal through the circuit, but I am uncertain about how to do this and what I should expect to find.If I construct the LC network consisting of components C14, C22, C23, L2 (in the second figure) separately, is there a way that I can check this network is working properly? Should there be a measurable signal if I probe at what would be pin 13? Do I need to drive the network with something to test it is working ok?

author: vk6zgo
author_rank: Super Contributor
timestamp: January 30, 2016, 03:40:58 am
content: Unless you have a Signal Generator,you won't see anything much at pin 13 .The signal from a FM Broadcast station is going to be so small at that point,as to be indiscernible.This Device is a Superheterodyne,so possibly your Local Oscillator isn't working.You will be able to probe pins 5 and 6 for this signal,as it is much higher in level.Don't expect a lot in level accuracy,as a X10 'scope probe doesn't look high impedance at 100MHz,but you should seesomethingon one of these pins if your Oscilloscope is 100MHz or better bandwidth.There seems to be no adjustable RF tuning,so you just tweak the LO to select stations--a 'scope will show if the LO tunes OK.I also notice there is a "mute" function built into this chip,which could be operating.

author: helix1
author_rank: Contributor
timestamp: January 30, 2016, 11:02:14 am
content: At the moment, none of the pins are giving any signal.Would it be worth building a RF transmitter that I could put close to the receiver so that I could get a measurable signal that I can follow through the circuit to see where it is going wrong?Or would it be more helpful to put an amplification stage between the LC input circuitry and pin 13/14?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 30, 2016, 11:32:06 am
content: Quote from: helix1 on January 30, 2016, 11:02:14 amThe VCO (pins 5,6) should be running independently of any input signal. If there is nothing there the chip is broken or you have an error in your design.Can you post some pictures of the board?

author: helix1
author_rank: Contributor
timestamp: January 30, 2016, 02:54:49 pm
content: Here are a couple of pics of the circuit as it stands... red is Vcc, black is earth, green is signal out and yellow is the antenna.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 30, 2016, 05:08:50 pm
content: It is hard to see how it is connected, but if you built it exactly as the circuit I can at least see one big problem:

author: helix1
author_rank: Contributor
timestamp: January 30, 2016, 09:24:11 pm
content: Yes this is a mistake on the TDA7000 datasheet. My circuit was built according to the first diagram in the original post, which doesn't have this error.I'm currently looking how to create a minimal layout so that I can check the local oscillator...

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 30, 2016, 09:52:28 pm
content: Quote from: helix1 on January 30, 2016, 09:24:11 pmIt could well be that the LO stops working if you put a scope probe on one of the pins, so keep that in mind.What test equipment do you have? A quick sanity check would be to see if the current draw is what it should be according to the data sheet.Another thing to consider is that you are working with 100MHz frequencies and that long wires will screw things, a better way to build this would be dead bug style on the copper side of a pcb.

author: KE5FX
author_rank: Super Contributor
timestamp: January 30, 2016, 10:53:34 pm
content: Just hold another FM radio near it to see if you can hear the LO.

author: helix1
author_rank: Contributor
timestamp: January 31, 2016, 09:16:32 pm
content: I have a scope and multimeter, and I made the RF probe detailed inAm I correct in thinking that the RF probe shouldn't affect the LO like a usual scope probe might?I'll investigate some more and see what I can find. Will also try rebuilding the circuit on a copper clad board. Thanks for the suggestions!