2011-02-24

ME4-T 70MHz Transverter

ME4-T transverter

The first  pieces of  the 4m  transverters we built on base "OZ KIT's by permission of the supplier.
We was very dissatisfied with very poor thermal stability of the LO.
By idea of the  good  quality OE9PMJ(†) 6m transverter we constructed our new 4m transverter.
We modified it on some points, (LO, switching unit, inner DC stabilizer, IF amplifier, and added possibility of the PWR control on
the front panel. We constructed new double sided PCB and inner switching and switchable attenuator unit up to 5W to the 28MHz IF.
On the DP-6  LED bargraph PWR meter unit you can check easy the output power.
The 25W out PA unit constructed by datasheet and recommendations of  Mitsubishi RA30H0608M RF module. The ME4-T
mechanics, the box and the outlet is 100% our own design.
 

Local Oscillator

The LO  is working with 10ppm 14MHz low phase noise xtal oscillator. +/- 10ppm  stability is enough for good quality FSK mode work.
The output signal of the LO is about 14dBm, it drives two 7dBm ballance TX and RX mixers over attenuators.
The output signal of the LO is clear the 2nd harmonics is lower than -60dB.
For better stability the oscillator chain is supplied with 9V from a 78L09 voltage regulator.
The correct LO frequency we can set easy with L17.

RX

The 69.9-71 MHz input signal passes through the input filter to the input amplifier (BF988) what the gain is approx. 20 dB. Then the
signal goes through the band pass filter providing a suitable selectivity. The balanced mixer MX1 mixes the 69-71 MHz signal down to 27-29 MHz loosing
approx. 6 dB in the process.  The  IF signal is amplified approx. 10 dB in a low noise high current J-FET (P8002). The band-pass filter 
increases the selectivity considerably. The noise level of the RX is typ. better than 2dB.

TX

The transmit mixer, MX2 only needs approx. -18dBm 28 MHz IF signal from the transceiver. A suitable level can be achieved by adjusting
VR1 in the attenuator. Lot of transceivers  has lower or much higher IF output level. To solve this problem we built in
an additional jumperable  and variable 5W/-20dB attenuator in the new versions of ME4-T. You can switch on/off  easy the attenuator with different jumpers.
The fine level setting possible with the poti P1 on the attenuator unit over the slots of the top cover.
The 70 MHz TX signal behind the MX2 mixer is  filtered through a three-stage band-pass filter  before being amplified in a BF982, and finally
a BFR96TS  to a level exceeding more than 300 mW. Through the final pi-filter we can reduce the harmonics of theTX signal.

The PTT circuit uses two BD136 for the RX/TX , switching TX when the PTT is grounded. At the PTT out terminal (+12V/1A) is available during TX.
It is intended to the antenna relay and the external PA SND transistor (BD237 - is on the rear attenuator/switch panel) and/or to the optional 2x60mm dia heating fans. The circuit has sequncer: the BC547 delays the TX key while activates the antenna relay immediately and also the external PA relay. This means that the TX output  is delayed approx. 100 ms after the antenna relay is activated.  The antenna relay of the ME4-T and the external PA switches without any TX signal present.
We constructed >25W output amplifier to the base transverter unit. It is built on with RA30H0608M Mitsubishi module by Mitsubishi datasheet and applications.
The output signal of the transverter drives the RF module over -10dB attenuator. We set the RF module to AB1 class by a simple circuit applicated by Mitsubishi.
The output signal can reach the 4m UHF type ant connector accross a harmonics filter and a built in relay.
The final unit has MONI output to check the output power.


 

LED bargraph Unit

To check the output power we using a simple LED bargraph unit with LM3419 integrated circuit. On this PCB we found also the high current DC relay.
This relay is switching the Vpp voltage to the PA unit. The calibration of the output power is possible with VR2 poti on the PA PCB.













 

Switching/attenuator panel

 We constructed inner switch/attenuator unit instead of old MET5/20 external attenuator. Lot of people want to use different IF level transceivers from
home or / portable. The jumperable attenuator can solve the problem on case high level or low lever output transceivers.
You can set ON or OFF the 5W attenuator, and  you can select easy between  single or dual cable IF operating with  the built in jumpers.
The unit consists  a switching circuit to  the PTT  external PA's (red colour RCA, SND output,( can switch max +50V,200mA)










 

Position of built in attenuator jumpers

If you using single IF cable between your radio and the transverter you connect it to transverter IFin BNC.
On this case the IFout connector is not in use.
Don't forget to connect the PTT cable between radio send connector and ME4-T PTT input! Otherwise  the IF power kills the transverter.
Never use the attenuator more than 5W IF level!

Construction

The  transverter is built on a 1,5 mm double sided glass-fibre epoxy PCB is fitted into a standard metal sheet box measuring 148 x 74 x 30mm.
PA unit is fitted into 148x55x30mm standard box.
The external box of the transverter is constructed from 1mm painted iron plate. The heatsink is 150x65mm ALU  heatsink material.
If you using the ME4-T  continuously FSK mode you can order optional fan module (2pcs 60x60mm DC fans on plate).
On the front panel we can find the ON/OFF switch, the PWR poti (can reduce the output power back to 5-6Watts to external PA's) and the LED bargraph
power meter.