Delayed AGC implementaion and Noise figure measurement of Receiver with AGC
I have two questions:
(a) What is delayed AGC and how it is diffferent from simple AGC. What kind of AGC is generally used in BPSK/QPSK RF receivers. Simple AGC or delayed? I need some good reference material or schematic diagram of Delayed AGC circuit.
(b) While measuring Noise figure of a receiver, is it necessary to deactivate the AGC loop? When I am measuring noise figure of receiver with AGC in operation then noise figure value from the noise figure meter is coming out very large(around 20-25dB) but when AGC is not in operation then the noise figure is coming out to be around 4dB. Where is the problem? Can anyone help in this regard?
Regards,
Akrcal
A delayed AGC is one where the gain control is not activated until a given signal threshold is reached.
Not sure about BPSK of QPSK receivers, nut i wouild guess that 'simple'AGC would be used so that the detector is presented with a relatively constant amplitude.
I'm sure a Google search will provide plenty of information on delaye AGC circuits.
When measuring noise figure you need a linear system from input to output or the measurement wil be in error as you have found out. The AGC will compress the change in level between noise source on and off increasing the apparent noise figure. Always turn AGC off and use as low ENR noise source as possible to prevent any overloading of the receiver. It looks like your receiver has a noise figure of about 4dB
You could use the AGC voltage as an indication of the noise figure if it has to be enabled all the time. This does make the assumption that the gain of the receiver is stable enough.
Dear G4BCH,
Thanks for your reply.
You have mentioned the following:
"You could use the AGC voltage as an indication of the noise figure if it has to be enabled all the time. This does make the assumption that the gain of the receiver is stable enough".
Can you elaborate this point a bit?
Regards,
akrcal
Hi Akrcal
The AGC voltage is an indication of received signal strength, is quite commonly used for this.
To be accurate the receiver gain must be stable otherwise the gain control will be adjusted to keep the output constant to compensate for gain variation rather than input signal variation. For an accurate measurement there are also problems with the way that the AGC detector responds to noise as compared with CW.
Injecting a noise source at the front end as a test signal during known dead time is quite often used as a part of system monitoring.
Peter