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Receiver sensitivity decreased

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raiya_23

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Hi I am working on a 332kHz receiver whose sensitivity has decreased it is receiving a signal of upto 20uV but wont receive a signal lower than that, can anyone guess what could be the problem?

I checked the RF amplifier and audio amplifier ICs in the circuit and they are perfectly fine.
 

chuckey

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" but wont receive a signal lower than that, can anyone guess ". Oh yes it can, the problem could be that there is an amplifier some where that is not allowing a lower level signal to get "through". Its not got the squelch level set incorrectly, if you mentioned the form of modulation it might help.
Frank`
 

raiya_23

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I checked the amplifier voltages. The amplifier ICs are working correctly and its not a transeiver its just a receiver so there is no modulation going on, only demodulation is taking place

I have observed that it starts receiving a signal lower than 20uV if I tune the variable inductors (The tuning circuit is made of ceramic capacitors and ferrite core variable inductors) however there is a lot of noise in the background alongwith a slight signal tone below 20uV. I need to bring the sensitivity to at least 5uV.

Note: The squelch setting is through these variable ferrite core inductors. This is a 29 year old receiver btw.

" but wont receive a signal lower than that, can anyone guess ". Oh yes it can, the problem could be that there is an amplifier some where that is not allowing a lower level signal to get "through". Its not got the squelch level set incorrectly, if you mentioned the form of modulation it might help.
Frank`
 

ALERTLINKS

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There is lot of noise and interferences generated by other appliances in this band making it unusable. Locate such a device nearby.
 

chuckey

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""on, only demodulation is taking place", is it FM, AM, MCW, SSB,. . . ?
" The squelch setting is through these variable ferrite core inductors.". Squelch action is only dependent on the level of AGC. If the signal is not strong enough, the AGC voltage will reflect this and a threshhold control will mute the receivers output. It is not dependent on tuned circuits, other then to de-sensitise the receiver.
Frank
 

raiya_23

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AM modulation is being used and there isnt a AGC circuit present as it is written in the manual:

"Since this receiver does not use an AGC circuit, the RF gain control must be adjusted to place the detector output level at a point of the detector characteristics that is linear."

""on, only demodulation is taking place", is it FM, AM, MCW, SSB,. . . ?
" The squelch setting is through these variable ferrite core inductors.". Squelch action is only dependent on the level of AGC. If the signal is not strong enough, the AGC voltage will reflect this and a threshhold control will mute the receivers output. It is not dependent on tuned circuits, other then to de-sensitise the receiver.
Frank
 

raiya_23

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But it receives a signal lower than 20uV with lot of background noise and so far I havent been able to make it receive a signal lower than 12uV

So, tune the inductors and problem is solved.
 

chuckey

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Right, the standard (?) sensitivity test is to use a carrier modulated at 30% with 1KHZ or 400 HZ to give a 10dB signal+noise to noise ratio. So apply this signal to your receiver and adjust all the gain controls for maximum sensitivity, remove the signal generator and replace with a 50 or 75 ohm termination. Connect a meter with a dB scale across the loudspeaker. Take a reading, this will be the residual noise. Apply the signal generator and adjust the level for a 10 dB increase on the meter. What the signal generator reads is the receivers sensitivity. As a precaution, adjust the signal generator for a higher level signal, say +6 dB, ensure the meter reads + 6dB as well, just to make sure that the AM receiver is not limiting or its AGC is coming on prematurely. The input signal should be able to handle +30 dB increase in its input signal level before the AGC starts to reduce the gain, else the signal to noise ratio will always be poor.
Frank
 

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