Construction and General Help

VK3XDK Transverter  - Construction notes.

 

The following is a guide to construction. I use the same techniques on all units.

PCB MOUNTING.

Using a strong solvent, clean both the PCB and aluminium carrier thoroughly prior to gluing.

Mix just enough Silver Epoxy for an even thin spread across the aluminium.

Press the PCB down onto the glue and use a bit of gentle rotation to sit the PCB flat and even on the Epoxy.
Place assembly on a flat even surface (under a bit of weight) until the Epoxy is at least half set.

Clean up any excess using a strong solvent (I use Paint Thinners but Acetone should work).

PCB SOLDERING.

Clean top surface ready for soldering.

MCX connectors need their centre pin trimmed to sit flat (surface mounted), SMA centre pins may also need shortening (depending on track length)

Solder the MCX connectors before you fit the Mixer chips because the excess heat could damage the fragile Mixer.

Sit the MCX connector in position and solder the centre pin to its pad first. This will hold the connector in place while soldering the body.

Use a hot iron or even 2 small irons to get the heat into the MCX body fast.

Solder the Mixer quickly and carefully. It is FRAGILE and hard to fault find. BE CAREFUL!

It is recommended that you use very fine solder with a very fine Soldering Iron tip.

A magnifying glass and some brush on liquid Flux makes this job a lot easier (hardest part of the board construction)

Soak up any solder “bridges” etc. with Solder Wick (be gentle, don’t use too much heat!)

Double check the soldering with a magnifying glass !
Fit the SMD parts leaving the Semiconductors until last.

First melt a bit of solder on one side of the PCB pad only, use tweezers to position the component then re-melt the solder to secure the component in place.

There are several paralleled components. I align the two next to each other on the bench. Then using tweezers, grip together and solder in the above manner !

Liquid Flux helps lots with this type of soldering/resoldering

Form and trim “leaded” components for surface mounting.

SMA CONNECTORS.

Temporarily solder the SMA's in position (centre pin only!)  Once mounted, the bodies can be rotated (gently) for alignment.

Mark out SMA hole positions using a fine tip marker.

Remove the SMA’s.
Centre punch and drill to suit mounting screws (I use small self tappers)

Be very careful while drilling into the edge of the aluminium (for obvious reasons!)

Refit SMA’s and solder permanently.

 

BEFORE POWERING UP.

Attach the DC Supply wires for +8 RX and +8 TX. You will need to supply an “off board” Regulated +8 Volt Supply Rail. If you don’t have any 7808’s, the simplest way is to “UP” a 7805 or similar TO-220 Flat Pack Regulator.

Check all work! Check for shorts!

And then… check all work AGAIN ! After 20 (more like 50) or more boards like this, I still make silly mistakes.

Adjust the 20K Bias Trimmer Pot for about 800 Ohms to Earth.

This should supply enough “safe” Bias for the HJFET for the moment!

 

RX POWER UP

Low Noise FETS can be unstable. Although I’ve not had problems with these designs, it is good practice to apply 50 Ohm loads to all RX ports before power up.

Adjust the Bias Trimmer Pot for 2.0 Volts Drain-Source.

Apply an Oscillator Injection Signal of appropriate level (typically +6 to +7 dBm) into the Multiplier. If you have access to a Spectrum Analyser or Power Meter, check the final Multiplier Output level. There should be +10 dBm on each Port. Otherwise, if it’s OK, you should hear the IF Noise floor rise !!

If you have access to a Sig Gen, apply a relatively “loud” RF Signal of say -80 dBm and check the IF Output. For the most part, Snowflaking will NOT be necessary but you may recover a few extra dB of Sensitivity.

There should be at least 8 dB of Conversion Gain for these transverters. If not, check the DC levels (marked on schematic). If these are all OK, there may be a problem with the Mixer. They are fragile and cannot easily be tested. Keep an eye out for stability issues although from past experience, there shouldn’t be any. However, if you suspect instability, check all grounding etc. The little SMD capacitors or resistors could also be damaged.

As a last resort, try a bit of microwave absorbent material.

 

TX POWER UP

Apply the Oscillator Injection and IF Injection signals of appropriate levels into the Multiplier and IF Ports. These are typically +6 to +7 dBm for both Oscillator and IF.

There should be at least +7 dBm Output available but +10 dBm is more typical with the correct Injection Levels applied.  If not, check your DC levels (marked on schematic). If these are all OK then you may have a suspect Mixer. They are fragile and cannot easily be tested.

Keep an eye out for stability issues although from past experience, there shouldn’t be any. However, if you suspect instability, check all grounding etc. The little SMD capacitors or resistors could also be damaged.

As a last resort, try a bit of microwave absorbent material.

 

Graham Byrnes VK3XDK  (grumss@yahoo.com.au)