hello,

After completing my AF amp and BFO, I'm in the process of doing IF/BFO
diode ring mixer.
I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

I found one from http://www.qrp.pops.net/dip2.htm

My question is: how on earth should I wind those 10mH inductors? My
calculations show that I should wind 487 turns on FT37-43 (which is
obviously impossible), or 437 turns on FT50-43 (which is hard..but
should be possible).

Any other ideas how to make 10mH inductors for AF diplexer?

thanks!

wrote:
hello,

After completing my AF amp and BFO, I'm in the process of doing IF/BFO
diode ring mixer.
I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

I found one from http://www.qrp.pops.net/dip2.htm

My question is: how on earth should I wind those 10mH inductors? My
calculations show that I should wind 487 turns on FT37-43 (which is
obviously impossible), or 437 turns on FT50-43 (which is hard..but
should be possible).

Any other ideas how to make 10mH inductors for AF diplexer?

thanks!

The best solution is a pot core. A molded inductor is ok, too, but you
might have trouble with hum pickup -- the voltage produced by any field
coupled into the inductor will be amplified by the whole audio chain. A
pot core is self-shielding like a toroid.

Roy Lewallen, W7EL

wrote:
My question is: how on earth should I wind those 10mH inductors?

In years past I would pick up 88mH/22mH and like "phone company"
toroids at hamfests etc.

My understanding is that these were used to compensate for line loading
in long phone wire runs.

Windings were either used as-is (if 88 mH or 22mH was appropriate) or
were removed and rewound.

Don't know what the actual core material was, I'm sure it was
completely inappropriate for RF but did fine at audio (I used them in
RTTY demodulators).

Haven't seen these on the surplus market in many years now. Too bad,
just about every 70's-era ARRL handbook had a half-dozen projects based
around them. A quick look at Digikey turns up some 2 through 16 mH
toroids intended for use in switching power supplies (page 1619).

Tim.

"Tim Shoppa" wrote in message
oups.com...
wrote:
My question is: how on earth should I wind those 10mH inductors?

In years past I would pick up 88mH/22mH and like "phone company"
toroids at hamfests etc.

My understanding is that these were used to compensate for line
loading
in long phone wire runs.

Windings were either used as-is (if 88 mH or 22mH was appropriate)
or
were removed and rewound.

===========================================

88 mH loading coils were used on 300-3,600 Hz phone circuits ever
since around the years 1900 to 1910. Spaced every 2000 yards in
542-pair underground, long distance cables between phone exchanges and
towns and cities, they were an international standard of loading.
Countless millions were manufactured. 88 mH coils must have
outnumbered all coils of all other values of inductance put together.
Originally the core was a bundle of soft iron wires but in the 1950's
after 50 years they changed to ferrites.

22 mH coils were another popular value. They were used on "music"
circuits. Spaced every 500 yards the frequency range was 100-15,000
Hz. At one time used on a network between radio broadcast studios and
transmitters.

Coils had to be removed to make cables suitable for carrier telephony
and then digital transmission. But there must be millions still in use
around the World.
----
Reg.

wrote:
hello,

After completing my AF amp and BFO, I'm in the process of doing IF/BFO
diode ring mixer.
I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

I found one from http://www.qrp.pops.net/dip2.htm

My question is: how on earth should I wind those 10mH inductors? My
calculations show that I should wind 487 turns on FT37-43 (which is
obviously impossible), or 437 turns on FT50-43 (which is hard..but
should be possible).

Any other ideas how to make 10mH inductors for AF diplexer?

thanks!

I would be looking for a core with bobbin to make winding a lot
easier. Something like this:
http://cgi.ebay.com/FERRITE-TRANSFOR...mZ7597936903QQ
Is an over sized core, but would work. One 1/4 this size would also
work. I would add a thin gap (sheet of paper) between the core
halves, to make the inductance stable. You can chuck the bobbin in a
variable speed drill, by passing a bolt through it, and wind hundreds
of turns in a couple minutes. You can adjust the inductance with the
core gap, if you have a way to measure the inductance.

wrote:

I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

My question is: how on earth should I wind those 10mH inductors?

I'm curious, is the virtual ground at the input of a modern
high-bandwidth op-amp (set up with unity or only moderate gain) good
enough at frequencies of concern that terminating the mixer into this
with a resistive network would produced the needed lack of RF
reflection?

I know that in terms of the audio frequencies, op amps with R and C
passives have long been a viable option to big inductors - but can they
now satisfy the need to terminate the RF components?

wrote:
wrote:

I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

I know that in terms of the audio frequencies, op amps with R and C
passives have long been a viable option to big inductors - but can they
now satisfy the need to terminate the RF components?

I've been playing with this a bit in simulation, and came up with
something seems to work. Most likely it is either common knowledge, or
fatally flawed - would be interested to hear which.

What I started with is an op amp with a grounded non-inverting input, a
50 ohm resistor from the mixer output to the inverting input, and a 50
ohm feedback resistor from the op amp output to the inverting input.
With an ideal op-amp, the inverting input would match the non-inverting
input's ground with a virtual ground, and the mixer would be
resistively terminted into this at 50 ohms for all frequencies.

Two obvious failure modes come to mind. First, at some frequency the
op-amp feedback will no longer be able to maintain a satisfactory
virtual ground. I tried to address this with a capacitor from the
inverting input to ground. 220 nF seems to clamp it there below the RF
frequencies where a 1 Mhz gain-bandwidth op amp fails, but not roll off
the audio response too badly.

Second, in the presence of a strong intefering signal on the order of
volts, the op-amp probably can't drive enough current through the 50
ohm feedback resistor to reproduce this, so non-linear effects (and
intermodulation) may set in. I attempt to address this by shunting the
feedback resistor with a 470nF capacitor, to make the gain roll off
towards zero above audio.

There is some slight reflection at the transition frequencies, but it
looks like it can be kept very low. One major weakness of this circuit
is that the rolloffs are largely first order and thus gradual. Given
that the goal of the exercise was to avoid the use of large inductors,
one improvement might be to precede the mixer with some higher order
selectivity at RF or IF, where inductors, or even ceramic resonators
are easy to come by. Another obvious improvement would be to pad the
output of the mixer down by quite a bit, though that probably means
picking up some broadband noise from the op-amp when it is reamplified?

So far I've only played with this in simulation. Trying to think how I
would test it if I were to actually build it. I have some access to a
network analyzer and would guess I could use that to measure the
reflection from the input that the mixer would see over a wide
frequency range. Measuring the ability to handle a weak signal in the
presence of a strong interfering one might be harder.

wrote:
hello,

After completing my AF amp and BFO, I'm in the process of doing IF/BFO
diode ring mixer.
I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

I found one from http://www.qrp.pops.net/dip2.htm

My question is: how on earth should I wind those 10mH inductors? My
calculations show that I should wind 487 turns on FT37-43 (which is
obviously impossible), or 437 turns on FT50-43 (which is hard..but
should be possible).

Any other ideas how to make 10mH inductors for AF diplexer?

thanks!

Just buy some Toko 10RB 10mH inductors. I've use them in that type of
diplexer and they work very well. They are available in the UK from
Rapid Electronics and other suppliers.

73, Leon

Hi,

I had exactly the same issue a few months back. Winding so many turns
on a FT50-43 is not realistic. Actually, winding the same number of
turns on any core, even the pot cores recommended by other people are
not so good for this application. The issue is the parasitic
capacitance. In my case, I needed ~2mH inductors. After I wound the
required number of turns on FT50-43, which was about 60 (IIRC), then
measured the parasitic capacitance, it was too much. The self resonance
was somewehere in the below 1 MHz. The 60 turns just about fits onto
the core. If you go for second layer (and beyond), the capacitance will
skyrocket.

A diplexer is supposed to provide close to real, near 50 Ohm impedance
to the mixer. At the same time, it needs to work as a low pass filter,
to keep the RF out of teh audio stages. If you do some simulations of
the common audio diplexer topologies and take into account the
parasitic capacitance of the inductors, you'll find out that the match
is close to 50-Ohms real only at audio, maybe up to LF. Completely
useless at HF and above.

So what's the solution? A few options:
1/ Try a material with higher permeability. Toroids are best, because
they don't need shielding and they don't break as easily as pot cores.
I'd recommend FT50-J. This is I think the highest permeability that
Amidon sells. You should end up with much smaller number of turns. It
may improve things in your case.
2/ Forget diplexer. Use an attenuator, followed with a low pass filter.
This will increase the noise figure & gain of the mixer-post-amp combo,
so you'll have to make it up with more gain ahead of the mixer. But
you'l have higher IP3. You could set the amount of attenuation to
balance the noise figure against IP3. Where the optimum is, I have no
idea.
3/ Since you are mentioning BFO & IF, I assume that you have a
superhet. The mixer at the end of the IF strip does not have such hard
requirements, since you've already filtered the 'bad guys' out. For
SSB, you would not care about intermod performance after IF filter too
much; well maybe except for audio distortion. Actually, for a product
detector in a superhet for something like SSB, don't even use a diode
mixer. Too much power and not necessary. Switch over to an active
mixer, like the NE602.

Anyway, your mileage may vary. The above should work for common cases.
If you really need a proper broad-band RF diplexer which also works as
an audio low pass filter, then you're in for an interesting journey...

Hope this helps,
Fil
VK3FLP

wrote:
hello,

After completing my AF amp and BFO, I'm in the process of doing IF/BFO
diode ring mixer.
I understood that it is a good idea (or even mandatory?) to follow DBM
with a diplexer.

I found one from http://www.qrp.pops.net/dip2.htm

My question is: how on earth should I wind those 10mH inductors? My
calculations show that I should wind 487 turns on FT37-43 (which is
obviously impossible), or 437 turns on FT50-43 (which is hard..but
should be possible).

Any other ideas how to make 10mH inductors for AF diplexer?

thanks!