On Wed, 18 May 2005 08:54:23 -0700, "RST Engineering"
wrote:

1) Transistors for RX front end: relatively easy to find if based on
dual-gate MOSFET

OK, but you can get better noise figure from a GaAs part.

True, however most uses for 2m a good transistor or fet will be
adaquate and likely easier to use. GaAs fets are harder to use
and can easily take off at microwave and harder to find.

For real fast lashups I've used MAR or ERA (MiniCircuits) MIMICS
and they are the easiest way to get good stable and predictable
gain at VHF through microwave and a kit of 30pieces was
around 49$ (ERA 1,2, and 3types). Parts can be had for
under 2$US in singles. Compared to transistors, FETs and
GaAs devices they are noiser but for a lot of things a MAR6
at 3db and 20DB gain to 1000mhz is not shabby.

2) RX band filters: I have decided to have a fixed capacitor and adjust
the inductor.The availability of adjustable coils (Toko for example) has
decreased in the last few years. Digikey only stocks a few of them but
that should be OK in the near term.

Mouser stocks a VERY nice line of low cost VHF shielded inductors. Google
"xicon" and "tunable coil" on the Mouser website.

There are many suppliers. For 2m use it's possible to design with a
bandpass frontend easily and then tuning is less an issue. I just
wind coils and bend them as needed to tune. Programs to calculate
a coild for that are easy to find on the 'net.

3) First IF filters: not much choice since 10.7MHz and 21.4 MHz seem to be
the only 2 frequencies stocked (most commercial boxes use between 35 and
70 MHz). The IF inductors/transformers at 10.7 MHz and 455 KHz have
disappeared from Digikey (used to be there a few years ago).

However for those who care to it's possibel to roll your own. Ladder
filters using very common 20 or 24 mhz clock crystals are do able.

Again, Mouser is your friend -- google "xicon" and "transformer". If I were
doing it, I'd do a 70 MHz. first IF and then 10.7 MHz. second IF. Forget
dropping down to 455 unless you have to.

Dual conversion using Murata and other 455khz filters is easy.
Motorola (3357 and friends) and other have a complete dual conversion
IF (everthing after first mixer) in a DIP and all you need is 10.7,
CFU455 filters and 455 if can for the quadarture dectector and 10.245
mixer crystal. These are easily ripped from:

Baby monitors, (flea markets and garage sales)
49mhz hts (non supperregen) (flea markets and garage sales)
Old scanners (flea markets and garage sales)
Other older radios that have been 4d (4d diseased, dying, dead
or decaying). (flea markets and garage sales)

4) 2nd IF + FM demodulator. The Motorola parts (MC33xx) are gone. The part
that is found in many commercial XCVR is the Toshiba TA31136. This part is
not stocked by Toshiba's distributors in the US (12 weeks lead time if you
want to order 2K of them). Good news is that the Rohm BA4116FV is a
pin-to-pin compatible device and it is available at Digikey for $1.26. I
don't know if this part works since I have not seen it used anywhere...

There are suppliers selling NOS motorola parts:

http://www.danssmallpartsandkits.net/ has them sometimes
http://partsandkits.com/parts.asp has the 3362 at 2 for 4$ (good
source for toroids, NE602 and other bits).

Phillips makes (made??) a nice line of FM ICs. You might give them a try.

The 604 can be hard to find.

6) 2nd IF filter. The 455 KHz ceramic filters seem to have a very limited
availability. The exact type recommended by the IC vendor is hard to
find..

Often thats easy to substitute and wider (15kc) ones are easy to find.

Why crystals? You are going to have a synthesizer for the front end anyway,
why not use the synthesizer crystal as your timebase and synthesize the
mixer oscillator? You said you wanted some stuff for the microprocessor to
do anyway; controlling a synthesizer is one of them. Or, you could get a
parallel input synthesizer and hardwire it to a fixed frequency.

The only way to go unless you doing a repeater or other fixed
frequency application. One caveat, it's far easier to create a clean
crystal LO chain and for super low noise frontends the LO must also
have narrow noise sidebands.

7) FM discriminator. I could not find a source for the ceramic
discriminator recommended by Toshiba/Rohm (CDLBLA455KCAY24-B0 which is the
new P/N for the CDB450C24). I can understand the pain of the distributors
since every FM IC requires a different discriminator. The discriminator
coil that can be used instead is not easier to locate.

A discriminator (or ratio detector) is nothing more than a couple of
transformers coupled bottom to top. You can make a two-coil ratio detector
as easy as buying one.

True, the ratshlock PRO4, Pro12, series and the Pro7 series scanners
used a trnsistor 455khs IF and two common IF cans for a discriminator
with excellent results. The transistors were of the 2n2222/2n3904
equivelent class easily gotten form JDR at 4-6 cents each.

You can do a quadrature detector using a single IF can and a NE602
mixer.

11) TX low pass filter: will need to be hand made. That's OK but who is
selling tinned wire ?

Why would you need tinned wire? Plain old magnet wire is easy to come by.

Yep! tinned is not a requirement. Besides of you do it right you not
going to need much.

I've used MAR6 mimic (cheap from Minicircuits) as RF with a TUF1 DBM
feeding a MPF102 fet with it's output running into a textbook 3357 if
chain for both 2m and similar for 440 and it's clean, easy and plenty
of gain (sub .12 uv for 10db quieting FM). The Moto 3357 came from a
49mhz HT along with the 10.245 crystal, 10.7 filters and 455filter. I
used a moto 145170 pll for the LO. Since the 3367 also does metering
and squelch all it needed was audio (LM386-N). The front end was low
loss (1db) bandpass before the MAR6 and image stripping (Higher loss
bandpass) after the MAR6. the DBM can be one of a dozen different
parts had cheap or even hand made (1n914s work ok at 2m). The post
DBM fet was not optimized for gain only to buffer the DBM from the
Crytal filter (10.7mhz).

Of course of you want something as a kit Ramsy has a FM rx for
(FR146C is 2m, they have 10m, 6m and 220) that can be used for that
range and has fair rx performance but needs either crystal control
(NE602 VFO is not adaquately stable) or PLL either of which is usable
with that. (it's cheap too! 44.95$US). They do about .7 uV for decent
quieting and they do also sell a RF preamp thats decent. I've built
both and they work as expected using the VFO and better with crystal
control.

For experimentors its possible to do it. Parts even old ones are
often found from small suppliers at resonable prices. Like someone
said earlier learning where and what to look for is a big part of home
made radio systems. It also helps to learn how to strip and use "old"
parts.

Allison
KB1GMX