IF AMP Queston

[realInfo]

Hi all

what is the "right" amplification of IF amps ?

How this factor is best calculated in a superhetrodyne receiver ?

Thanks
EC

 

[MooseFET]

Hi all

what is the "right" amplification of IF amps ?

How this factor is best calculated in a superhetrodyne receiver ?

AM receivers have AGC to adjust the gain up and down. FM radios have
limiting.

 

[[email protected]]

AM receivers have AGC to adjust the gain up and down.  FM radios have
limiting.

To expand on this a bit:

Typically you want just enough RF gain prior to your mixing stage to
maintain a good S/N ratio. Typically this is going to be about 20 dB
of gain. But you do not want too much RF gain (for a robust receiver
- always exceptions to the rule) because of front end compression
issues.

As a result, most of your gain will be in your IF section, but becuase
the incoming signal can vary in strength (assuming you want a high
dynamic range receiver) then you can vary the gain (or limit it in FM)
as stated above. (Or, you can use a log amp for pulse detection ).

 

[[email protected]]

Hi all

what is the "right" amplification of IF amps ?

How this factor is best calculated in a superhetrodyne receiver ?

Thanks
EC

Say you want to pick up aerial signals down to ~1uV.
You'll be wanting to listen to these on a speaker or headphones, so
say ~1V of audio.
That's fixed the maximum overall voltage gain for the complete radio
to 1 million.
The audio amp' might have a gain of 10, hence the total RF gain needs
to be ~100,000.
Sounds a lot but gain is cheap. The clever bit is using it with loving
care.

 

[john jardine]

The audio amp' might have a gain of 10, hence the total RF gain needs

F***,Sh**. "RF" wrong. Read "IF". (Rare to find RF gain).

 

[[email protected]]

F***,Sh**. "RF" wrong. Read "IF". (Rare to find RF gain).

A put down is not very effective if nobody knows what you are talking
about.

 

[Leon]

A put down is not very effective if nobody knows what you are talking
about.

IF amplifiers used in HF receivers often have a gain of about 60 dB.

Leon

 

[Leon]

F***,Sh**. "RF" wrong. Read "IF". (Rare to find RF gain).

RF gain is very common on high-spec. receivers, like the Redifon R551N
I have.

Leon

 

[[email protected]]

A put down is not very effective if nobody knows what you are talking
about.

Twas same person berating himself

 

[Grant]

Rule of thumb: the IF amp has to have enough gain to compensate for the
losses of the mixer(s) and filter(s) in the IF stage, but not much more.

For best overall noise figure, the amplifier gain closest to the receive
antenna is the most important. It should be high enough to make noise
contributions from the IF stages insignificant, but not so high that the
signal level will be too high for the first mixer.

IF amp gain around 20 dB or so is common.

 

[Leon]

Rule of thumb: the IF amp has to have enough gain to compensate for the
losses of the mixer(s) and filter(s) in the IF stage, but not much more.

For best overall noise figure, the amplifier gain closest to the receive
antenna is the most important.  It should be high enough to make noise
contributions from the IF stages insignificant, but not so high that the
signal level will be too high for the first mixer.

IF amp gain around 20 dB or so is common.

If you only have 20 dB IF gain, where is the bulk of the gain going to
be achieved? Something like 100 dB total is needed to amplify a 1 uV
signal to something that is audible. And what about AGC? It won't be
very effective with only 20 dB to play with.

Leon

 

[MooseFET]

.
.
Really?  Decades of engineering work out the window.  Most of us haveshot
for somewhere between 50 and 80 dB and found that span decent to work with.
Sorry, the RF amplifier stage sets the noise figure to a first
approximation.  All the stuff behind is is second order at best.

I would have put it as the RF amplifier and mixer but once past there
it is more like a second order approximation that the noise numbers
are set.


sqrt(1^2 + 0.5^2) = 1.12

If your RF stage only gives you 6dB of gain it dominates the noise
issue.

Yeah, I know.  Deep space picovolt work requires something on the orderof
100 dB (what a crystal bitch to tame)

The earth bound end has the advantage of being able to spread things
out. The amazing stuff is the stuff that has to be kept light enough
to launch.

and crackerbox AM radio can get by
with 40 dB, but 20 dB is laughable.

An old tube AM radio I fixed many years back had less than 40dB in the
IF. The gain was something like 45 or 50. It could receive about the
same stations as I could get on my crystal radio. Its only advantage
was that you could hear them one at a time instead of all mixed
together. I think that even 40dB may be a little low for a good AM
radio.

Who is this idiot?  Wasn't he the one that was trying to adapt AM sideband
principles to FM modulation?  Somehow I don't think either Bessel OR
Armstrong is his friend.

Since the whole world is going digital, I'll give this one away:

If you single side band modulate the upper side band with a signal and
modulate the lower side band with minus the signal and radiate the two
sidebands as the carrier, you have a signal that can still be
understood if it get clipped along the way. It still sounds like hell
but you can make out what is being said.