Receiving Loop Antenna Question
 

In message , Richard Clark
writes
On Wed, 28 Jan 2009 20:12:15 +0000, Ian Jackson
wrote:

Thanks for the advice, however......

Why? A half volt signal level

(that's surely one hell of an RF signal?)

I can see you don't know what environment you are living in. That, or
you live out in a pastoral setting and the house has no significant RF
contribution. Most of the world is urbanized and folks live within
short distances of large transmitters.

In the UK, most medium wave transmitters - and certainly those which are
high power - are located well out of town. I'm 20 miles west of London,
and the nearest MW TX is about 25 miles away (20 miles north of London,
the area which it is intended to serve).

Varicap diodes will always suffer from having their capacitance
modulated by an RF signal impressed upon them. However, they do seem to
work well enough - even when the RF level is pretty high (which must be
the case especially with varicap-tuned oscillators - ie most VCOs and
PLL systems).

You don't put the Varicap into the high level part of the circuit.
This is obvious from the outset.

Surely it is at the high voltage part of the circuit where the tuning
capacitor will be most effective in tuning (ie most kHz per pF)?

Presumably, the effect of this modulation will be to generate
intermodulation products. In VCOs, this will simply appear as harmonics
of the oscillator signal (which you would get anyway - even with a
conventional tuning capacitor).

A conventional cap (and a conventional inductor, much less) will not
contribute harmonics because it is linear.

Well, of course it won't. And neither will be an inductor (especially if
it's air-cored, as in a loop antenna).

Harmonics comes from
nonlinear components added by the circuit (partial conduction of an
active component that aids in the oscillation).

Where multiple-frequency signals are
present (like you have with the receiving loop), the most apparent
effect of the modulation of the diode capacity will appear as
crossmodulation and other nasties on the other signals in the passband.
However, as varicaps ARE used for the tuning of the input of receiver RF
stages, how do they 'get away with it'?

They don't if you are in such an environment. Again, if you don't
know your environment, then it's all a crap shoot. There is no
"getting away with it." The alternative is that the designer of a
product fully anticipated these issues and purposely chose a design
that minimized the effect of accidental contributions.

That's what I am interested in finding out. Just how DO they make
varicap diodes work adequately in (say) receiver front ends?

Not all
designers are up to speed on the topic. The web is full of reported
failed projects that do not take bias Z and filtering into
consideration.

This swing is also a problem for PIN diodes used as voltage variable
attenuators.

My understanding of things is that the effectiveness of PIN diodes
relies on them having a very poor performance at RF (especially at VHF
and UHF). In attenuator circuits, they are forward biassed, and the DC
current passing through them varies their RF resistance. However, the
charges flowing through the junction are so 'sluggish' that they don't
react to the rapidly-changing RF voltages. There is therefore negligible
modulation of the RF resistance by the RF signals passing though them.

Poor performance when a PIN diode is first and foremost a diode for RF
and microwave applications? Only if you are using it for the wrong
reason (like using a 1N23 for 120V 60HZ power rectification).

My main personal experience with PIN diodes has been limited to their
use as 'variable resistors' in matched-impedance variable attenuators in
cable TV equipment. The essential characteristic is that there is a
chunk of ordinary (intrinsic) silicon between the P and the N layers. If
the RF signal alternates quickly enough, the electrons don't have time
to travel across the intrinsic layer before the polarity changes. As a
result, the diodes don't rectify. The higher the frequency, the better
they get at 'not rectifying'.

PIN diodes are used as resistors and switches, not rectifiers.

See above.

Further, don't confuse the switch application as meaning fast turn-off
in the pico-to-subnanoseconds. Speed is relative to the application
of signal path steering where 100s of nanoseconds is more than
adequate.

While I've used ordinary diodes as RF switches, I can't remember if I
have used PIN diodes. I think that, at the RF levels involved in CATV,
an 1N4148 junction diode worked just as well or better.

Consult:
http://www.ieee.li/pdf/pin_diode_handbook.pdf

Thanks. Yes, I'll have a look, but it's how to use varicaps that I
really want to know about.
--

Receiving Loop Antenna Question
 

Ian Jackson wrote:
. . .
That's what I am interested in finding out. Just how DO they make
varicap diodes work adequately in (say) receiver front ends?
. . .

Very carefully. One of the main tricks is to severely limit the tuning
range by using fixed capacitors, switched as necessary, to make up the
bulk of the circuit capacitance. This reduces two serious problems,
nonlinearity which causes distortion, and temperature sensitivity.
Harmonic distortion can be mitigated by following the tuned circuit with
filters (provided that the total tuning range is narrow enough), but
this doesn't correct intermod problems caused by the nonlinearity. A
third potential problem if you use varicap for wide tuning range is that
the control voltage has to be heroically filtered to prevent any noise
or ripple from modulating the capacitance. The problem of nonlinear
tuning is often solved by using a control loop, for example an AFC to
detect whether the desired signal is properly tuned and to automatically
adjust if it isn't.

I remember one of my first encounters with a varicap, where I naively
used it as the bulk of the circuit capacitance in a homebrew VHF-to-BCB
converter. I was leaning over the prototype, trying to find a signal,
but hearing only hearing blip. blip-blip-blip. blip, sounding like
signals going by as if a tuning dial was being spun. Then I realized it
was synchronized with my breathing. Every time I exhaled,
blip-blip-blip-blip. Then when I inhaled, they'd go by a little more
slowly, the other way. That temperature sensitivity, incidentally, is
extremely difficult to compensate, since it's different at every reverse
bias setting. The only way to have a chance is to swamp the varicap C as
much as possible with fixed C, then approximately compensate the
remaining temperature drift.

Roy Lewallen, W7EL

Receiving Loop Antenna Question
 

Roy Lewallen wrote:
Ian Jackson wrote:
. . .
That's what I am interested in finding out. Just how DO they make
varicap diodes work adequately in (say) receiver front ends?
. . .

Very carefully.

Might it help to use strings of varicaps? Sure I've seen that
done somewhere.

Receiving Loop Antenna Question
 

"Richard Clark" wrote in message
...
On Wed, 28 Jan 2009 20:19:20 GMT, "Jerry"
wrote:

Hi Richard

I made a capacitator rotator for my 6 foot AM loop using a hobby shop
servo to rotate the air variable cap. It works quite well and the
components are affordable.
The 3 section air capacitor allowed the use of each section by switching
to the appropriate section with a TO-5 relay.

That is Brute Force to tune an AM loop, but sure makes sense and uses
affordable parts.

Jerry KD6JDJ


Hi Jerry,

You have certain advantages over most of the correspondents here. You
are something of a gear head, you know where to find things, you know
how to put them together, and you actually do it.

When I was a kid, I had the same bent, but my resources were from
government surplus when you needed 400Hz 26V supplies (or other such
oddities). It was back in the 60s when I started experimenting with
Varicaps except they were 1N23 style diodes sold for their Varicap
properties (which comes free with almost any diode); the real Varicaps
sold for far too much for my allowance. Wasn't much tuning ratio back
then either - about 3:1 to 5:1 - today's have more range than most air
caps.

73's
Richard Clark, KB7QHC

Hi Richard

I worked for Hoffman Labs in 1960 and had responsibility for designing and
developing a sweep frequency receiver for the US Navy. I used a varicap
to sweep the L.O.. I am not inclined to use a varicap where a lumped
capacitor will do the job.

Jerry KD6JDJ

Receiving Loop Antenna Question
 

Clifford Heath wrote:
Roy Lewallen wrote:
Ian Jackson wrote:
. . .
That's what I am interested in finding out. Just how DO they make
varicap diodes work adequately in (say) receiver front ends?
. . .

Very carefully.

Might it help to use strings of varicaps? Sure I've seen that
done somewhere.

The only advantage I can see is that it would increase the amount of
distortion for a given level of signal, since the amount of capacitance
change with signal voltage would decrease. The tradeoff would be a
reduction in the capacitance and an increase in the required tuning voltage.

Roy Lewallen, W7EL

Receiving Loop Antenna Question
 

Roy Lewallen wrote:
Clifford Heath wrote:
Might it help to use strings of varicaps? Sure I've seen that
done somewhere.
The only advantage I can see is that it would increase the amount of
distortion for a given level of signal

Decrease, I think you mean. And yes, that's what I was thinking of.

Receiving Loop Antenna Question
 

On Thu, 29 Jan 2009 01:05:33 GMT, "Jerry"
wrote:

Hi Richard

I worked for Hoffman Labs in 1960 and had responsibility for designing and
developing a sweep frequency receiver for the US Navy. I used a varicap
to sweep the L.O.. I am not inclined to use a varicap where a lumped
capacitor will do the job.


Hmmm, a swept frequency receiver for the Navy. Sounds like something
the spooks would listen to. Crypto or surveillance gear?

73's
Richard Clark, KB7QHC

Receiving Loop Antenna Question
 

"Richard Clark" wrote in message
...
On Thu, 29 Jan 2009 01:05:33 GMT, "Jerry"
wrote:

Hi Richard

I worked for Hoffman Labs in 1960 and had responsibility for designing
and
developing a sweep frequency receiver for the US Navy. I used a varicap
to sweep the L.O.. I am not inclined to use a varicap where a lumped
capacitor will do the job.


Hmmm, a swept frequency receiver for the Navy. Sounds like something
the spooks would listen to. Crypto or surveillance gear?

73's
Richard Clark, KB7QHC

Hi Richard

You figured that out rather well and quickly too.

Jerry KD6JDJ

Receiving Loop Antenna Question
 

On Thu, 29 Jan 2009 06:23:32 GMT, "Jerry"
wrote:


"Richard Clark" wrote in message
.. .
On Thu, 29 Jan 2009 01:05:33 GMT, "Jerry"
wrote:

Hi Richard

I worked for Hoffman Labs in 1960 and had responsibility for designing
and
developing a sweep frequency receiver for the US Navy. I used a varicap
to sweep the L.O.. I am not inclined to use a varicap where a lumped
capacitor will do the job.


Hmmm, a swept frequency receiver for the Navy. Sounds like something
the spooks would listen to. Crypto or surveillance gear?

73's
Richard Clark, KB7QHC

Hi Richard

You figured that out rather well and quickly too.

Jerry KD6JDJ


Hi Jerry,

I trained with CTs (Communication Techs, otherwise those who
maintained and used Crypto or surveillance gear). When they couldn't
figure out a technical problem they always called the Cal Lab (we had
total responsibility for anything electronic).

After the capture of the USS Pueblo, it was said by my buddies that
Marine sentries were posted at the entrances to the comm room - "just
in case."

Of course, this was a lead they offered so I would ask "To protect
them from the boarders?" to which they would say "No, the Marines
would go in and shoot the CTs." This was similar to my own days
aboard the USS Holland, which, when it went out to sea was accompanied
by a submarine whose reputed orders were to sink us if we were in
jeopardy of being taken over. We had a technical library full of
systems plans for the Boomers.

I also served with pig-boat submariners who spoke in hushed tones
about the USS Thrasher and Scorpion. Then there was my time with the
CBs who had spent more time in Nam than any Marines.

There was a lot of Black Humor in those days.

73's
Richard Clark, KB7QHC

Receiving Loop Antenna Question
 

Clifford Heath wrote:
Roy Lewallen wrote:
Clifford Heath wrote:
Might it help to use strings of varicaps? Sure I've seen that
done somewhere.
The only advantage I can see is that it would increase the amount of
distortion for a given level of signal

Decrease, I think you mean. And yes, that's what I was thinking of.

Yep, sorry, I did mean decrease. Thanks for catching it.

Roy Lewallen, W7EL