I'm playing with a homebrewed receiver design and need a LO which
covers 120 to 240 MHz. What's the feasibility of using something
like the MAX4545 to electronically switch the ADF4360-8 VCO's
inductors in order to cover that range? What other approaches
are recommended for a homebrewed digitally controlled LO which
covers that range?

-- John
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| Feith Systems | Voice: 1-215-646-8000 | Email:
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| John Wehle | Fax: 1-215-540-5495 |
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John,

We were just having a similar discussion on sci.electronics.design, although
it was about switching in capacitors on the ADF4360's L1/L2 pins rather than
different inductors. One of the comments -- from Michael Terrel -- was that
he's worked on equipment that shorted out inductors at various taps to change
ranges. You might get some ideas from reading the discussion:
http://groups.google.com/group/sci.e...cdc83a3cf9a3d5

One problem with the ADF4360 is that the DC bias is, of course, ground on both
ends of the inductors so the RF swings above and below ground, thus requiring
either fancy bias arrangements or devices such as MOSFETs or PIN diodes that
will conduct on both cycles with a single-ended control voltage.

wrote in message
ups.com...
I'm playing with a homebrewed receiver design and need a LO which
covers 120 to 240 MHz. What's the feasibility of using something
like the MAX4545 to electronically switch the ADF4360-8 VCO's
inductors in order to cover that range?

Well... the tuning range is 1.2:1, so you'd need at least 4 -- probably 5,
more practically -- switched inductors to cover the range. I suspect you
could make this work, although the "grounded tapped inductor" approach is
probably a little cleaner.

What other approaches
are recommended for a homebrewed digitally controlled LO which
covers that range?

DDS chips! (E.g., AD9858.) The main drawback is that they consume
significantly more power than a VCO/PLL combination and none will directly
synthesize frequencies much above the low UHF range. But for 120-240MHz on
mains-powered equipment, DDS would work well.

---Joel

wrote in message
ups.com...
I'm playing with a homebrewed receiver design and need a LO which
covers 120 to 240 MHz. What's the feasibility of using something
like the MAX4545 to electronically switch the ADF4360-8 VCO's
inductors in order to cover that range? What other approaches
are recommended for a homebrewed digitally controlled LO which
covers that range?

-- John
-------------------------------------------------------------------------
| Feith Systems | Voice: 1-215-646-8000 | Email:
|
| John Wehle | Fax: 1-215-540-5495 |
|
-------------------------------------------------------------------------

(ADF4360 L's don't appear accessible?)
Done a bog standard, single transistor Colpitts, that covers 110-220Mhz
using a cheap SMV1255-000 varicap. It's part of a BFO arrangement and
intentionally limited to a 110MHz swing (1.5 to 4.5V) but there's a bit of
steam left in the Varicap and 120-240 seems not unreasonable. Present plan
is to feed the VCO to a LMX2306 synth run as always, by a PIC.
(BFO design started after rooting round for a now unobtanium /100 frequency
count prescaler for another project and noticed the cheap LMX2306 can act as
all things to all men ).
john



--
Posted via a free Usenet account from http://www.teranews.com

John,

"john jardine" wrote in message
...
Done a bog standard, single transistor Colpitts, that covers 110-220Mhz
using a cheap SMV1255-000 varicap.

What's the power consumption of the VCO? (I'm looking at breaking apart one
of those ADF4360's into a separate VCO and PLL, since the 4360's use a *lot*
of power, but apparently most of this is tied up in the VCO since you can
readily get PLLs that are only a milliamp or two such as your LMX2306...)

"Joel Kolstad" wrote in message
...
John,

"john jardine" wrote in message
...
Done a bog standard, single transistor Colpitts, that covers 110-220Mhz
using a cheap SMV1255-000 varicap.

What's the power consumption of the VCO? (I'm looking at breaking apart
one
of those ADF4360's into a separate VCO and PLL, since the 4360's use a
*lot*
of power, but apparently most of this is tied up in the VCO since you can
readily get PLLs that are only a milliamp or two such as your LMX2306...)



2ma at 5V.
It's a 2N3819 fet with 1k5 source. Runs about 300mV rms (or, as this is a
radio group +2.57dBm For curiosity, the original fet circuit seemed not
unhappy to be pushed to 500MHz, (on plug in breadboard!).
Noticed the "-7 -8" ADF4360 versions use external Ls. Nice feature (to me)
is that the oscillator is routed internally to drive the DivN counters. At
the moment am using 2 MMC's to give a 1Vrms (apologies, +13dBm) final output
to drive other circuits, ALC and the synth but they burn power like there's
no tomorrow.
john



--
Posted via a free Usenet account from http://www.teranews.com

john jardine wrote:
Done a bog standard, single transistor Colpitts, that covers 110-220Mhz
using a cheap SMV1255-000 varicap. It's part of a BFO arrangement and
intentionally limited to a 110MHz swing (1.5 to 4.5V) but there's a bit of
steam left in the Varicap and 120-240 seems not unreasonable.

That's a Kv of about 36 MHz / V. Naive question ... doesn't a higher
Kv
have the potential for greater noise? What is the effect of a higher
Kv
for the LO VCO on the overall performance of a receiver?

Joel Kolstad wrote:
One of the comments -- from Michael Terrel -- was that
he's worked on equipment that shorted out inductors at various taps to change
ranges.

I can understand using a MOSFET to short an inductor at various taps,
however
I don't see how to do that using a PIN diode unless DC blocking
capacitors are
being used between the inductor sections.

I looked at this chip some time ago for use as the LO in an HF receiver,
and I abandoned it concluding that the noise performance of the VCO is about
20 dB shy of the minimum you need for good HF receiver overload performance
('reciprocal mixing").

As to the VCO tuning range, less is usually better from a noise standpoint,
as most varactors don't have very good Q specs and the tighter the varactor
is coupled into the overall resonant circuit, the poorer the resonant Q will
be. Also, high tuning sensitivity in the VCO makes the oscillator
susceptable to modulation from 'junk' on the power supply or the tuning
voltage.

Shorting out turns in the resonant inductor as a means of bandswitching
isn't so great either, as the shorted turns inductively couple into the
resonant circuit as resistors (the resistance of the pin diode or other
switching element), lowering the circuit Q and raising the noise of the
oscillator.

In general, it's best to band-switch discrete elements such as separate
inductors or capacitors and to keep the tuning sensitivity as low as
possible in order to provide continuous coverage of a band. If high tuning
range is necessary, every attempt should be made to use high Q components
and to maintain a high loaded Q in the circuit they're used in. Also,
parallelling of varactors has been shown to improve noise performance in
wide-range oscillators.

Joe
W3JDR


wrote in message
oups.com...

john jardine wrote:
Done a bog standard, single transistor Colpitts, that covers 110-220Mhz
using a cheap SMV1255-000 varicap. It's part of a BFO arrangement and
intentionally limited to a 110MHz swing (1.5 to 4.5V) but there's a bit
of
steam left in the Varicap and 120-240 seems not unreasonable.

That's a Kv of about 36 MHz / V. Naive question ... doesn't a higher
Kv
have the potential for greater noise? What is the effect of a higher
Kv
for the LO VCO on the overall performance of a receiver?

Hi Joe,

That's some really good information; thanks. One question...

-- If shorting part of an inductor to ground is not that desirable since the
PIN diode or other switching device will have finite resistance that'll create
noise, isn't there the exact same problem with switching in multiple discrete
devices? That the switching device will create a little noise?

---Joel

W3JDR wrote:
I looked at this chip some time ago for use as the LO in an HF receiver,
and I abandoned it concluding that the noise performance of the VCO is about
20 dB shy of the minimum you need for good HF receiver overload performance
('reciprocal mixing").

Are you refering to a strong signal acting as a local oscillator and
when mixed with the phase noise sidebands of the actual local
oscillator
producing results which fall into the IF being used?

The phase noise performance of the ADF4360-8 is listed as -120 dBc/Hz
at 100 kHz offset from carrier. The MAX2620 which I was also
considering
is listed as -110 dBc/Hz at 25 kHz offset from carrier. What did you
end up
using and what are the specs?