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1-8 of 8 messages
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Post-mixer amp ideas?
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by K8AI on May 31, 2008
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Anyone have a proven circuit for a post-mixer amp? I'm having a problem with a 80m single conversion superhet. I'm using a SBL-1 mixer and a MC1350 IF amp
with a 4-xtal, 9MHz xtal filter.
I would like to find something other than the classic 2N3855 circuit.
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RE: Post-mixer amp ideas?
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by WB6BYU on May 31, 2008
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What are your goals for third order intercept and/or noise figure for the
receiver? What operating voltage do you have available? What sort of
current draw are you aiming for?
A common base/gate amplifier is often used to provide a 50 ohm
termination to the mixer. Generally speaking, the more current you
stand in the amp the higher the intercept point will be. This is why they
sometimes use power transistors such as the 2N3866. I'm sure I've seen
some that use the IRF510 in this application as well. If you aren't as
worried about the strong signal handling characteristics then you can
get by with a smaller transistor and less current draw.
Another possibility is a variation on the Norton amplifier. You can find
some information about it here:
http://www.qrp.pops.net/preamp.asp
There should be a good discussion about post-mixer amplifiers in any
of the standard references, such as Solid State Design for the Radio
Amateur or Experimental Methods in RF Design. I've also got some of
the Technical Topics Scrapbooks from the RSGB and can look for a
specific circuit if I know what parameters you are looking for.
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RE: Post-mixer amp ideas?
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by VE3EFJ on June 2, 2008
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A DB(diode)M must be terminated properly. It does make a difference. The common broadband feedback amplifier using a 2N3866 or 2N5109 is typical, but the parts count is a bit high.
It is also important that the LO port have sufficient drive.
An MMIC is an alternative, and a MAR-4 is often used. Dan's small parts can help you there.
If you don't use a broad band amp, then you'll need a diplexer and that gets a little ugly (again) in the parts count area.
What I've found to be very good is a grounded gate J310 amplifier. Gain is about 10 dB and the input impedance is easily matched from a 2:1 toroid from 50 ohms (Input of about 200 ohms). You'll only get this gain from a high impedance load on the drain, but thats easy to do.
I've used both a MMIC and the J310. Either has worked well. The 2N3866 broad band amp also does a fine job, if you don't mind the parts count. You don't need a lot of gain. Drake used a J310 in the TR7 following the DBM, so that should give you an idea of what is achievable.
Wayne
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RE: Post-mixer amp ideas?
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by K8AI on June 3, 2008
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Thanks for the help you guys. I don't have any target performance figures planned, I'm not an established homebrewer yet. I would just like to actually FINISH a project someday!
I'd like to finish a few projects and one is this 80m single conversion RX (as part of a 30-50W transceiver). So far, I have a stable and clean VFO with +7 dBm out, bandpass filter, SBL-1 mixer, 4-xtal filter, 1 and 2 stage MC1350 IF amp (I've tried both), a SBL-1 product detector with 9MHz BFO (also, +7 dBm out), and a LM386 AF amp.
I've checked my filters on the HP 8920 and they're working properly. The RX is quite deaf thus far.
Do I need a diplexer after the mixer(s)? If I need a post-mixer amp, other than a MAR-4, what else could I use? I have some ERA-1,2 and 3's and some others.
Curt, K8AI
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RE: Post-mixer amp ideas?
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by WB6BYU on June 3, 2008
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The mixer should be terminated in 50 ohms at all frequencies
(or at least for the IF and major image frequencies) to get
reasonable overload performance. You can do this with an
attenuator pad (adds loss), with an amplifier that has a
wideband 50 ohm input impedance, or with a duplexer that
terminates the undesired frequencies in 50 ohms and lets
the IF see the input impedance of the following amplifier
(or crystal filter).
Is your receive deaf due to low gain or due to high noise
level? I've seen some circuits using the MC1350 or other
ICs that have a high noise level that hides weak signals
even though it has good gain. (Though the MC1350 isn't
the worst offender, and should work OK with enough gain
in front of it.) One approach is to work through the
receiver with a signal generator to see how much gain
you have in each stage.
Do you have an RF preamp ahead of the mixer? A diode
mixer has insertion loss, and while a super low noise
figure isn't needed in most cases on HF, you probably
will want a low-noise amp either before or after the
mixer and before the crystal filter (which also has
loss). The grounded-gate JFET amplifier is a good
choice for a post-mixer amp: a simple part like the
MPF102 or 2N3819 may be adequate for testing, but for
good overload performance something like the J310 with
a higher Idd current would be a better choice. (The
U310 is the same die mounted in a package that is designed
for grounded-gate service because the case can be
soldered directly to the ground plane, but you probably
won't see a significant difference in performance at HF.)
When I'm prototyping a receiver I build each stage
on a separate square of circuit board and wire them
together on the bench. This makes it easy to replace
one of them with an alternate circuit and see what
difference it makes. Once you find a combination that
works you can either mount all the boards into a case,
or (if space is at a premium, as it is for my hand-held
receivers) rebuild the receiver on a single board.
If your gain is low you might also consider adding a low-noise
audio preamp following your product detector. If you
are using a diode mixer here you have the same issue that
it needs a good termination, and again a grounded-base
stage (as seen in several circuits by W7ZOI or KK7B)
provides good termination and low noise figure.
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RE: Post-mixer amp ideas?
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by KB1GMX on June 3, 2008
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If you not looking for lot of gain but to terminate the mixer well I'd try a common base J310 FET. That can work well with low noise and good termination that will not be
affected by the filter [high reverse ioslation]. Gain for a stage like that will be in the 8-12db range typically. It also has the advantage of good intercept for a moderate operating current.
Allison
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RE: Post-mixer amp ideas?
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by K8AI on June 5, 2008
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I've got some J310's coming. I found some time to construct a bridge-T diplexer after the SBL-1. I now need to at least try a norton amp using a 2N3904 to see if it makes a difference and if not, I'll try a preamp in front of the SBL-1.
Yep, I'm constructing each stage on it's own little board and using the HP 8920 SA/tracking gen to get the passband response of each stage. Thanks again.
Curt, K8AI
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RE: Post-mixer amp ideas?
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by WB6BYU on June 5, 2008
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You can start by experimenting with a grounded-base
2N3904 circuit while you are waiting for your J310s.
That should also give a good wideband termination if you
set the standing current to give a 50 ohm input impedance.
It probably won't have the strong signal capability you
want for the final version, but you'll get a sense of
how much it improves the receive gain and sensitivity.
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