Looking for low-frequency PIN diode

[[email protected]]

I’m developing a simple, five-frequency superhetrodyne receiver to operate between 2.5 MHz to 20 MHz in which the local oscillator crystals and whipantenna tuning circuits are selected (switched) using diodes. Here’s theproblem:

The whip tuning circuit is comprised of a single variable capacitor with five fixed-value inductors, each in series with a BA277 "bandswtiching" diode.. A single diode/inductor combination draws about 6 mA when ON (switched in). This works fine, but at and above 15 MHz, the BA277 diode significantly contributes to the series resistance of the inductor, inducing a 3dB "insertion loss" (as compared to the same circuit with the diode short circuited)..

The BA277 datasheet indicates that at 6 mA and 100 MHz, the series resistance of the diode should be much less than 1 Ohm. No performance data is given for any other frequencies. My bench measurements indicate that at 15 MHz,the diode presents much more resistance than that (probably tens of Ohms).No surprise there.

QUESTION: The BA277 works OK for what I’m doing, but is there a bandswitching or PIN diode out there better suited for operation below 30 MHz?

-Dave

 

[George Herold]

I’m developing a simple, five-frequency superhetrodyne receiver to operate between 2.5 MHz to 20 MHz in which the local oscillator crystals and whip antenna tuning circuits are selected (switched) using diodes. Here’s the problem:



The whip tuning circuit is comprised of a single variable capacitor with five fixed-value inductors, each in series with a BA277 "bandswtiching" diode. A single diode/inductor combination draws about 6 mA when ON (switched in). This works fine, but at and above 15 MHz, the BA277 diode significantly contributes to the series resistance of the inductor, inducing a 3dB "insertion loss" (as compared to the same circuit with the diode short circuited).



The BA277 datasheet indicates that at 6 mA and 100 MHz, the series resistance of the diode should be much less than 1 Ohm. No performance data is given for any other frequencies. My bench measurements indicate that at 15 MHz, the diode presents much more resistance than that (probably tens of Ohms). No surprise there.



QUESTION: The BA277 works OK for what I’m doing, but is there a bandswitching or PIN diode out there better suited for operation below 30 MHz?



-Dave

Dave, I don't know much about what you are doing.
But isn't the dynamcial resistance of the diode the thermal voltage dividedby the current?
so ~25mV/6ma ~ 4 ohms
At 20 MHz you could use a swtich or a relay.

George H.

 

[Martin Riddle]

Motorola, GE & RCA used DC switching for crystals in business radios
for decades. You have to turn the diode on hard, or it causes all kinds
of problems. A relay is unreliable, if you need good accuracy. The
only manuals I have left are for synthesized business radios.

Looks like standard 1n914's were used in SSB radios for the Upper
lower and AM inductors in single crystal PLL radios. It was actually a
1S2075K, similar to the 1n914.
The PLL crystals were ~11Mhz or so. Not sure of the diode current,
doesn't look like more than 3ma.

Cheers

 

[Phil Allison]

"Jeff Liebermann is a radio ham Dope ".

Yep. You're probably measuring the diode lead inductance.

** LOL !!

The BA277 is an SMD device in a tiny SOD523 pak.

http://www.nxp.com/documents/data_sheet/BA277_N.pdf


..... Phil

 

[Phil Allison]

"Jeff Liebermann = trolling FUCKHEAD "

Ok, so lose the clip leads.

** Lose the FUCKING ASSHOLE ATTITUDE !!!

You LAZY, STUPID, PIG IGNORANT

RADIO HAM CUNTHEAD !!

And fucking stop posting stupid damn lectures

And then FOAD.




..... Phil

 

[Robert Baer]

I’m developing a simple, five-frequency superhetrodyne receiver to operate between 2.5 MHz to 20 MHz in which the local oscillator crystals and whip antenna tuning circuits are selected (switched) using diodes. Here’s the problem:

The whip tuning circuit is comprised of a single variable capacitor with five fixed-value inductors, each in series with a BA277 "bandswtiching" diode. A single diode/inductor combination draws about 6 mA when ON (switched in). This works fine, but at and above 15 MHz, the BA277 diode significantly contributes to the series resistance of the inductor, inducing a 3dB "insertion loss" (as compared to the same circuit with the diode short circuited).

The BA277 datasheet indicates that at 6 mA and 100 MHz, the series resistance of the diode should be much less than 1 Ohm. No performance data is given for any other frequencies. My bench measurements indicate that at 15 MHz, the diode presents much more resistance than that (probably tens of Ohms). No surprise there.

QUESTION: The BA277 works OK for what I’m doing, but is there a bandswitching or PIN diode out there better suited for operation below 30 MHz?

-Dave

Try a transistor,lower drop in inverse ("up-side-down") configuration.

 

[[email protected]]

I've seen power diodes like the 1N400x series suggested as HF PIN
diodes. You're using them "off label", so you can't be sure that you'll
get what you want in every one (at some point someone will decide that
last year's fast recovery diode exceeds all the specs and will start
printing "1N4005" on it -- then you'll be screwed). But it may be worth
a try.

IIRC only the 1N4007 (the 1000 V version) has some kind of PIN
action), the lower voltage versions don't.

 

[[email protected]]

How are you matching the whip antenna to this front end filter
(presumably an LP or BP filter) or is the "tuning circuit" just a
matching circuit? The short whip antenna is going to need a large
series inductor, which will tend to have low Q and high losses.

How long is the whip ? It is going to have a very high capacitive
reactance if it is less than 3 m (1/4 wave) at 25 MHz and much worse
at lower frequencies.

Is there a risk that a real outdoor antenna is to be connected to the
receiver or is it just whip only ?

There are several ways to connect an electrically short antenna into
the receiver.

One way is to put a equally large inductive reactance (loading coil)
in series with the actual whip and there is a pure resistance between
the antenna and the receiver input. However, the radiation resistance
for such short antenna might only be a few ohms, so still some
matching might be needed.

Considering the atmospheric noise levels, it might be a good idea to
have a resonant antenna (with loading coils) at 20 MHz, but at lower
frequencies, there are still going to be sufficient signals available
despite mismatch.

An other approach is to use some kind of source follower as a voltage
probe. The antenna capacitance of the whip (perhaps 10-100 pF) form a
voltage divider with the gate/source capacitance of the FET.

The voltage at the gate is then transferred to the resistor (or
emitter follower) at the source electrode, which can be loaded with a
typical 50 ohm receiver input impedance.

Of course, the source follower should operate with a sufficient
operating voltage (5-24 V), so that combined waveform from the antenna
does not clip.

This kind of construction is common in semiportable receivers with
antenna input for a real outdoor antenna as well as equipped with a
whip for portable operation. The outdoor antenna is connected directly
to 50/75 ohm antenna input, but when whip antenna is selected, the
whip and source follower drive the receiver input, instead of the
outdoor antenna.

What is the capacitance range of this capacitor ? If it is wide, then
at Cmax and large inductor at 2.5 MHz or alternatively Cmin and
smallest inductor at 20 MHz, the required inductance range is
reasonable. However, with a narrow capacitance range, a 1:8 frequency
range requires nearly 64:1 inductance range, which can cause problems
with strays and impedance levels.

Is this a mechanical variable capacitor or some varicap ?

If this is a mechanical capacitor, I can understand using only one,
but still I would consider switching in serial/parallel capacitors
(which of course reduces tuning range), instead of simply inductor
switching.

With varicaps, why not simply use 5 varicaps and have 5 independent
resonant circuits and select between these resonant circuits with PIN
diodes. In this way, there are no lossy switching element _within_ the
resonant circuit, which would dissipate the resonant current and hence
lower the LC circuit Q and hence increase bandwidth.

With separate resonant circuits, you can also tailor the impedance
values as needed for each band, considering also the (short) antenna
reactance.

 

[[email protected]]

Thank you for all the constructive suggestions. A few comments on your replies:

Regarding the BA277, the manufacturer (NXP Semiconductors) bills the deviceas suitable for “Low loss band switching in VHF television tuners” anddoesn’t explicitly call it a PIN diode. If it is one, then compared to most PIN diodes, its carrier lifetime must be fairly long if it can be usedat VHF frequencies (30 to 300 MHz).

I substituted the BA277 with 1N914 diodes in my prototype circuits and they’re definitely more lossy in the frequency range I’m working with (2.5 to 20 MHz). Therefore, the 1N914 and its equivalent, the 1N4148, are unsuitable.

In my application, the capacitance is constant at all frequencies. Consequently, as someone correctly pointed out, the reactance of the tuned circuitis low at 20 MHz and so the BA277 diode in series with the inductor noticeably contributed to the loss (lowers the quality factor Q).

Based on your posts, here are the two options I’ll consider:

1) Keep the whip antenna tuning circuit as is (single tuning capacitor, multiple inductors switched with BA277 diodes) and live with the loss. This keeps the circuit as simple as possible.

2) Redesign the tuning network to minimize the BA277’s contribution to the loss. This can be done with a capacitor (varactor) paired with each inductor, and switching these L/C pairs with BA277’s. This keeps the diode out of the L/C parallel combination.

Thanks again!

-Dave

 

[josephkk]

http://en.wikipedia.org/wiki/PIN_diode

The problem is to find a PIN diode with a low on resistance and a long carrier
lifetime, so it keeps conducting for a long time when current is flowingin the
back direction. If the signal is slow compared to lifetime, it acts likean
ordinary diode.

There's no carrier lifetime specified for the BA277. It's probably pretty short.

A little telecom type relay is a good idea.

Indeed. In a test lab we had no problem making 4-digit accurate and
repeatable (4-terminal) measurements multiplexing scores of devices in
thermal chambers with to-5 class latching relays and careful layouts,
wiring and fixturing at 10 MHz. Did have to be careful to use consistent
cable lengths (80 coaxial conductors and 196 pin connectors).

?-)

 

[Simon S Aysdie]

Thank you for all the constructive suggestions. A few comments on your replies: Regarding the BA277, the manufacturer (NXP Semiconductors) bills thedevice as suitable for “Low loss band switching in VHF television tuners†and doesn’t explicitly call it a PIN diode. If it is one, then compared to most PIN diodes, its carrier lifetime must be fairly long if it can be used at VHF frequencies (30 to 300 MHz). I substituted the BA277 with 1N914 diodes in my prototype circuits and they’re definitely more lossy in the frequency range I’m working with (2.5 to20 MHz). Therefore, the 1N914 and its equivalent, the 1N4148, are unsuitable. In my application, the capacitance is constant at all frequencies. Consequently, as someone correctly pointed out, the reactance of the tuned circuit is low at 20 MHz and so the BA277 diode in series with the inductor noticeably contributed to the loss (lowers the quality factor Q). Based on your posts, here are the two options I’ll consider: 1) Keep the whip antenna tuning circuit as is (single tuning capacitor, multiple inductors switched with BA277 diodes) and live with the loss. This keeps the circuit assimple as possible. 2) Redesign the tuning network to minimize the BA277’s contribution to the loss. This can be done with a capacitor (varactor) paired with each inductor, and switching these L/C pairs with BA277’s. This keeps the diode out of the L/C parallel combination. Thanks again! -Dave

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You're asking for very low resistance.

I've use the SMP1352 (Skyworks) before, but it only approaches 1 ohm. (1.5μs CL, 50 μm I-region)

You can arrange things at those frequencies such that by paralleling them (as far as AC goes), you get the benefit of "resistors in parallel."

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