Strange problem with link receiver (S/N ratio goes lower with audio)

[Elopid]

Hi guys,

A friend of mine is using an 'N' PLL band I link RX and there is a strange problem, I have a radio which shows both the main signal reading in dB's and also the S/N ratio in dB's.
But every time the audio is silent the S/N ratio meter dB's read much higher but when there is audio playing (particularly with bass) the S/N ratio dB's go down.

This never happened at all with their previous link RX which was a VFO version, but since they have changed it to this 'N' PLL version there is problems with the S/N ratio going down. It is also affecting the signal a lot worse as you get further away from the main TX compared to last time with the old receiver in what was actually a slightly lower power rig as well but was reaching out further as the S/N ratio was much higher.

Does anyone know what the problem is here? Is there something on the 'N' PLL link receiver which I can adjust for them to stop this happening?

All help is greatly appreciated.

[radium98]

post some pictures .

[Elopid]

I only have this one picture
Thanks in advance for your help

[RF-Head]

It's a M receiver
Do you use it in stereo or mono

[Elopid]

Hi "H"
it is always being used with Mono and the RDS level is perfectly set slightly lower than all the legal stations as is the audio which is set to around 65-70k deviation.
Do you have any idea what this problem is?
Another two receivers were used before this working perfectly with good high S/N ratio that never changed, but this receiver with audio the S/N ratio is going down from about 50dB to 35dB and going up and down with the music.

[sinus trouble]

Hello Elopid

Yes that is very strange? Unfortunately Recievers are almost impossible to tune correctly without test gear.

Just a Question? When you say Signal to Noise Ratio, Do you mean in terms of RF or Audio?

If Audio? Noise will obviously be unpleasant but would not necessarily effect the output power of the Rig.

If RF? Many factors such as atmospherics, Surrounding RFI influences and modulation level will effect the overall S/N.

As i guess you already know, A signal with no modulation is a focused and solid signal which is not the case once it is modulated.

My theory would be that it could be a Rig problem, Not the reciever? Or just some random phenomenon due to weather changes?...... Maybe LOL

[reverend]

It may depend on how the signal meters on the receiver work. One possibility I could imagine is that the receiver is somehow slightly off frequency. Imagine that your transmitter is on 52.0 MHz, but the receiver is on 52.1 MHz and that the bandwidth is 200 kHz (to make the maths easy). The transmitter frequency will vary from 51.9 to 52.1 but the receiver will receive from 52.0 to 52.2.

With no modulation, all of the carrier power on 52.0 MHz is in the receiver passband, but once you start to modulate it, some of the transmitted signal (that between 51.9 and 52.0 MHz) will be outside. If the receiver is measuring the total signal in its passband, when you turn the modulation on, the signal strength will appear to fall (note the same thing would result from over deviation, spreading the signal outside the receiver passband). Normally this would result in very distorted audio, but if the received signal is strong enough, it may not be noticeable.

Worth checking?

Rev

[Electronically]

When you say it's going up and down with sound levels sounds like to me that your mp3 music or cd recordings could be lower in sound to each track you play. For instance if you played a non copy of cd it's higher volume. Now if you put a copy of that disc in it would be lower. Just mp3 and cd recordings never are the same level of volume. Could this be your problem?. Or what to do is put a tone level throw it run variests tests.

Sent from my AMN-LX9 using Tapatalk

[Electronically]

With your other rig that you used was there more power output?. Just your pll might have slightly less power output. It needs to reaching its distance back n forth via the link.

Sent from my AMN-LX9 using Tapatalk

[radium98]

theremust be a fading somewhere or a drift .that what i canassume.what is the ic part number not to clear cx ?

most peaople say it for 90% correct ,without being at place it is a bit hard to focus 100%

i suggest to put at tx part a tone test for 1khz constant level near 0 db and carefully stay tuned at the rx part side and see what is going take the output of the rx to a line in aof a sound card pc and record it ,then open ann editor audacity etc ,and you can see what happen .

[Elopid]

Hi guys thank you all for your replies.

I know it has to be the receiver which is the problem and not the transmitter because the receiver that the transmitter had in it before was perfect and the signal to noise ratio level never dropped on the meter display, it is also perfectly tuned in to the correct frequency and is not off-tuned, the problem is it is quite badly affecting the signal strength in the further out areas
With the other link receiver it was much stronger signal coming out from the main transmitter and main TX was even around 30% less TX power than it is now

I just wonder, looking at that board, if anyone can suggest anything to do like adjusting something on there. I don't want to stuff it up so I don't just want to go twiddling things.
But it is seemingly making the signal it is receiving weaker when there is audio on the carrier, which I can clearly see on the signal to noise ratio on the FM radio as it drops, but when it is silent it rockets right up.

So there must be something on that board which is causing it to do this, I don't really uinderstand about link receivers but there is something to do with the audio maybe a screened toko coil or something, which is possibly affecting it as it receives the audio in.
I hope this makes any sense - the audio is adding a lot of noise to the carrier like a stereo pilot, but only with this link receiver, something is wrong.

Thanks All - ELo

[reverend]

What is the marking on the IF filter (the three legged blob that's on the left hand side of your picture just to the top left of the CA3089)? Is that a blue dot on it?

Rev

[radium98]

it must be a 10.7(A) or (S) mhz .but i am for 90% sure from the components around the C3089 specially if you have the toko of the discriminator out of frequency
try modulate the tx side at a low audio level ,and tell us what happened on the Rx side ( audio out )

[Elopid]

Thanks for suggestions guys
Unfortunately I am unable to look at it at the moment but I have managed to put together 3 pictures of the ceramic filter. This is the best I can do for now. It is hard to see the writing on it but hopefully you guys might recognise the blurry writing or the markings and work out what the number is on it.
I have never known this problem to happen with any other transmitter set up in the past, this receiver only.
eLo_

B1_RX_Ceramic_filter.jpg

[mpx]

It sounds like the oscillator might be slightly out of the bias range of the PLL, you could try turning the red screened toko coil (the one on the end at the opposite end to the crystal) slightly in each direction (just remember where you started, maybe make a mark on the ceramic slug with a bright coloured permanent marker and one on the casing to match it up). It would also help if the receiver is in a screened enclosure in case it is getting de-sensed by the PA

[radium98]

410

[Albert H]

It would also help if the receiver is in a screened enclosure in case it is getting de-sensed by the PA

A favourite housing for link receivers years ago was a 2oz tobacco tin!

[Elopid]

The receiver is housed in it's own seperate little box screwed on the side of the transmitter.
Albert do you have any idea what this problem could be, have you ever heard of this happening before?

/ELo/

[radium98]

i think the best is to make a little video ,so it can be seen what happened ,i know it can be hard ,but let say without rf power or few milli watt etc and good audio source .

[radium98]

i get my eye over the net o a beautiful band I link .