Dennis:
Noise and gain-bandwidth are neat concepts that not many people have
heard of or understand, but they are quite simple and very useful to know.
On this very subject, I was flipping through the 2001 handbook looking
for the R2 and saw that it is not there anymore. :( However, I did
find the "rock bender" receiver that someone else mentioned on the
list lately. It is a very simple DC receiver.
It uses a diode mixer as the detector, followed by a TL072 as the
first pre-amp (18 nV/SqrtHz). My first thought was that a lower
noise LM833 or a NE5532 (4 nV/SqrtHz) would be better served there,
but maybe that won't matter much on 40m.
Then I noticed the op-amp pre-amp feedback resistor was 1 meg, with the
50 ohms out of the mixer as the input resistor.
Hmmmm 1,000,000/50 = 20,000x gain (86 db of gain!). If the TL072 has
4 MHz of gain-bandwidth product, then 4,000,000/20,000 = 200 Hz. Therefore,
in this configuration, the poor op-amp can sustain a gain of 20,000x up to
200 Hz, where it then runs out of gas.
This means:
1) Lots of in band audio mixing (two cw tones will mix to form others)
and audio distortion because there is effectively no feedback above 200 Hz.
2) At 1 KHz, the most gain the op-amp will deliver is 4 MHz/1 KHz = 4000x,
so the audio is emphasizing signals below 200 Hz and rolling off signals
above 200 Hz.
Also notice that the majority of gain in the receiver (86 db) is placed
before the low pass filter section. This is not good. In order to have
good rejection of out-of band strong signals, you want as little amplification
as possible before your main filtering section.
My suggestion on this receiver is to use three op-amps instead of two.
Maybe a single TL071, and a dual LM833. Set the first opamp for a gain
of 100 (40 db), using one of the two low noise op-amps. Make the second
opamp the low pass filter as shown using the single TL071. Then after the
volume control, insert the second half of the LM833 set for 40 db of gain,
then follow up with the LM386 audio amplifier IC.
Look at before and after. Before, a strong signal out of band was amplified
by a factor of 4000x (1 KHz) before hitting the low pass filter. Since the
op-amp is maxed out, there is lots of audio distortion and audio mixing, and
this large signal may drive the op-amp into saturation (rail to rail voltage
output) before the safe haven of the low pass filter is reached and the big
out-of-band signal filtered out.
The poor little in band audio cw signal we are trying to listen to get creamed
by lots of bad stuff, much of which is generated internally by the receiver
itself.
In the "after" case, the large signal gets amplified by only a factor of 100
before it gets rejected by the low pass filter. Using a LM833 (15 MHz
gain-bandwidth) at a gain of 100, at 1 KHz, the pre-amp op-amp has an open loop
gain of 15 MHz/1 KHz = 15,000x. Since we used only 100x, there is an
excess gain of 150x or 43 db which is used as feedback to keep distortion
and mixing products under control. The large out-of-band signal then gets
filtered out by the low pass filter and the little QRP cw signal we wanted
to hear passes cleanly through to the following stages.
In addition, the lower noise LM833 will allow us to have another 12 db of
increased
sensitivity. I am not sure this is useful on the 40m band, but it is nice
to know that the noise you hear from the rig is band noise and not "designed
in" receiver hiss.
In addition, the volume control is now much earlier in the gain lineup. Having
86 db of gain in front of the volume control means that large signals in the
desired audio pass band can saturate the op-amps before we ever get a chance to
turn down the volume. Having only 40 db of gain before the volume control
greatly
reduces the chances of over loading the op-amps before we get a chance to
reduce
the signal level with the volume control.
This receiver as designed (the "before" case) has about 120 db of gain, 80 db
in
the preamp, 40 in the audio amplifier. This is about right for speaker levels.
Headphone level requires only about 90 db of gain. Therefore, if this were
used
with headphones only, I might pick the preamp to be 30 db (32x gain), the low
pass filter has unity gain, the second op-amp I introduced have another 32x
gain (30 db), with the audio amp chip, 40 db for a total of 100 db.
The point is that by adding one more simple audio stage and using better
op-amps,
this simple receiver can be improved greatly.
- Dan, N7VE
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