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Not strictly one of the RF Toolkits, but the following refers to the Multipig+ receiver, which shares a good deal of its circuitry with various RF Toolkits.
OK, here's the setup.
I'm using the multipig+ receiver board and have changed the IF frequency to 9MHz. (appropriate tuned circuits have been altered for the new frequency.......I assume the tuned circuits directly after the diode ring mixer.....diplexer (?) are tuned to the IF frequency?)
Ahead of the receiver board is a diode switched bank of band pass filters (one for each amateur band)
Ahead of this is the transmit LPF
So plenty of selectivity!
My antenna is a 3 ele triband yagi at 30'/10m.
The receiver seems to perform nicely.
The problem is at certain times of the day broadcast station breakthrough is terrible, the stations audible I think are on the 13MHz broadcast band.
Bands affected to some degree or another are 7, 10, 14 and 18MHz
Switching the RF preamp out of circuit seems to help, but does not seem to fully cure the problem.........the RX is then not sensitive enough!
If the local oscillator input is removed, the problem does not go away, in fact the BC signals seem louder and clearer (i.e. no QRM from those amateur signals!) so its dosent seem like a mixing problem.
I have tried using an ICE Ltd. Bandpass filter in place of my filter but have the same results, so the diode switching of the filters does not seem to be to blame.
The architecture seems the same as the elecraft K2, some Euopean users have had similar problems with their receivers experiencing the same effect........it has been suggested on their forum that the front end BPFs should be tightned up.
Any ideas anyone?? or is this something to be expected in the presence of high power BC stations here in Europe?
BTW my NE602/MC1350 superhet RX performs very well under the same circumstances with just a hint of attenuation in the front end.
73 Steve G4EDG
Are your 9 MHz crystal cases grounded?
Do you have a REAL GOOD GROUND and no ground loops?
Please describe "broadcast station breakthrough"...is it a full AM detected signal or sideband noise or something else?
What happens when you place the antenna at the input of the XTAL filter?
Have you tried to use a common mode choke on the power supply source to the RCVR?
73, Diz, W8DIZ
Thanks for your quick reply, to answer your points:
1. I have tried two types of IF filter (1) the on board crystals, all individualy grounded, as per the building instructions, and (2) an outboard set of filters (2.4KHz and 500Hz) mounted in a screened box, input and output fed with RG174, these are switched using the same method as in the TS830 i.e. diode swithced)
The problem exists to the same extent with both 1 and 2 above.
2. The multipig+ board is mounted on 4 x 10mm metal pillars on an aluminium chassis, as are the Band pass filters and IF filter box (when used) all are interconneced using rg174, screens are connected at both ends.
3. The quality of the BC breakthrough is excellent!! real good quality audio, if I listen through the 500Hz filter the audio bandwidth is restricted........so guess the signal is passing through the filter rather than around it.
4. With the input of the IF filter disconnected from Q6 (2N5109 amp) and connected to the antenna there is absolutely no breakthrough......just the noise I would expect to hear on 9MHz. If I isolate the input of Q6 and feed the antenna in there there is no breakthrough either, so guess Q6 is not the culprit.
5. I have tried a filter consisting of 20 bifilar turns on a ferrite rod in the power supply line......no change.
I wonder if there are any clues in the above Diz??
Can you determine the actual frequency of the SHort Wave station that is causing the problem?
Maybe the SW Station is mixing with another frequency source to produce a 9 MHz freq product?
OK, will do........although its a little difficult as the same station broadcasts on several frequencies simultaneously.
I did check the input to the filter with my scope and a rough sine wave at around 11MHz could be seen..........since then my 'scope has stopped working......I'd like to check again, I guess a spectrum analyser would be the tool to use.
I do live very close (around 250m) to a VHF FM broadcast transmitter (88-106MHz), and TV (around 500MHz) it also has some other services on it, though not sure of their frequencies.
I'll try taking the problem receiver to another location and see what happens
I did connect the input of the xtal filter to the output of the pre amp (Q1) at the centre pin of J4..........very strong break through. However if the link at J4 is removed i.e. disconnecting the diode ring mixer, the break through stops.........this is with no Local Oscillator input!!
Something has to be going on in that U2!!!!
I'll get my general coverage RX and calculator out and do some arithmetic.
73 Steve G4EDG
I'll go through what I did so you can follow my thinking.
I connected my FT-817 to the point where the multipig+ IF filter input goes (and disconnected the filter), with the '817 tuned to 9MHz (the IF I am using) thus using the 817 as IF strip.
This produced roughly the same results as the onboard IF i.e. massive broadcast breakthrough. It was interesting to listen with the 817 in AM mode.........several BC stations could all be heard at once all fading in and out independantly.
I noticed that adjusting the BFO/CIO on board caused the whole IF output to shift up and down.......which seemed strange as this should not have had any effect as I was using the 817 as the IF.
I killed the BFO/CIO by the base..........and hey presto the breakthrough ceased completely.
It seemes that the onboard BFO is leaking into the front end and being modulated by what ever strong signals have passed through the front end BPF......in this case the BC band around 13.7MHz.
My first thoughts were that maybe the CIO signal steering diodes (TX side) were leaky, so I removed them just to make sure........no improvement.
That +7dBm CIO signal just seemed to be getting in everywhere!!
I finally resorted to removing the onboard CIO crystal and making provision to replace this by using the oscillator circuit in the NE602 product detector........success!!!!!!!!!!!!!!!!!!
The receiver now sounds even better than before and the IF gain control is a lot smoother, no sudden rush of gain at a certain setting of the gain control.....now its a lot more gradual and controlable.
So thats the RX sorted out, now I need to study how to provide a +7dBm CIO for use on the TX side that is only enabled on TX.
How this modulation of the CIO takes place seems strange..............I think the RX preamp must be directly demodulating* all the BC signals at once producing an audio frequency spectrum which in turn is mixing with the leaking CIO signal to produce a real mess of a signal at the IF frequency........which the IF strip only to happy to pass and demodulate.
*I say this because the BC breakthrough virtually vanishes when the preamp is bypassed
Thanks for your help Diz
73 Steve G4EDG