XTAL-box-14M

1. Introduction

As the main problem in dense areas of many well equipped VHF contest teams appears to be TX noise originating in HF RIG a possible solution to this problem is TX IF preselector.

Although the main motivation for this project was HF RIG TX noise reduction, the IF preselector brings tremendous benefit on RX as well. Just imagine the "crazy" situation when the XVRT TX driving signal going out of HF RIG can be 20 dB lower than the RX IF signal going from XVRT into HF RIG receiver (50 km LOS, 2x 1kW and 17 dBi antennas pointed one to other ==> 10 mW signal level that enters HF RIG receiver, while XVRT drive signal is in the 0,1 mW range).

The preselector also has a major role in setting up an efficient remote RX system (RX system 50 ... 200 meters away from main TX antennas that can be used during transmit time). 

More about this was said at regular S5 VHF&up meeting in Nemčavci 2011 (also available in Czech - tnx to OK1UBO) and by other hams many many years ago (i.e.DL7QY in 1987). 

2. Block Scheme

Block sheme is shown on the diagram below and is very similar to the older project by Vlada/OK1VPZ.

3. Specifications

RX:

• Insertion Loss:      1,8 dB ± 0,2 dB
• P1dB:               >+13 dBm

TX:

• Gain:                10 dB (at 0 dB HF ATT)
• Gain regulation:     10 dB max
• Pout nominal:        +1 dBm (at 0 dB HF ATT)
• OIP3:               +23 dBm
• IMD3 / 5 / 7:       -46 / -72 / -90 dBc (at Pout 0 dBm)

General:

• Power supply:        13,8 V
• Consumption:         300 mA max
• Dimensions:          275 (W) x 163 (D) x 40 (H) mm

4. Construction

5. Description and Operation

Preselector has 13 filter selectrions: 8 for SSB filters, 4 for CW filters and one bypass. It is used on TX as well as on RX.

Insertion loss is less than 2 dB so it does not have significant impact on receiver sensitivity. Beacuse filters protect HF receiver from very strong signals the preamplifier on the HF RIG can freely be swithed on (AIP OFF on FT-1000mp). Preamplifier has to be switched on in case we add additional losses on the IF RX line (i.e. antenna IF switch matrix) to maintain required sensitivity of the complete receive system.

PIN attenuator and high dynamic range IF amplifier are placed in front of filters in the TX path. PIN attenuator functions as main output power setting control. TX amplifier replaces the IF TX AMP in the Javornik-144/14 XVRT. This means that IF TX AMP in XVRT needs to be switched off (position of jumpers J2, J3 and J4 need to be changed). By doing so about 6 dB lower TX noise of the XVRT can be achieved. Nominal TX level out of preselector is +1 dBm. This level is high enough to drive two XVRTs in parallel up to about 30 W (XVRT Pout potentiometer fully open!). In case output power from XVRT is not high enough the missing gain should be obtained by changing PI attenuator between BFG196 driver and power module (default 11 dB for 60W version) and NOT by switching-in IF TX AMP.

Two PTT signals can be connected to the preselector, »main« and »sub«. »PTT main« is normal PTT function – it switches both RX/TX relais. »PTT sub« function is supporting more advanced contest setups with two HF RIGs, where the second RIG (»sub«) is used for S&P. The »sub« RIG will be mainly outside the »main« RIG filter passband so »PTT sub« also switches »bypass relay« beside RX/TX relais. This way any transmission with »sub« RIG will always bypass filters.

Filters can be selected by two push buttons on front panel ( ← and →). Remote push buttons can be connected to the 3.5 mm stereo jack on the back panel. It makes sense to assemble 3 push buttons on a small PCB with long enough cable so that such a small control PCB can be freely positioned on the desktop area. Third, the middle, button should connect both signals to the ground in parallel - pictures below are self explanatory. Pressing middle button selects bypass position - pressing it again returns to the last selected position. It turned out that the middle button is being the most pressed button during the contest operation.

                                                                                                                                                                                        Remote filter selection buttons 

HF TX ATT (see block scheme) value depends on the HF RIG in use. Install 0 dB for FT-1000mp. Nominal TX input level is between -9 dBm and -11 dBm. In case HF RIG provides higher TX level, the difference needs to be leveled out by HF TX ATT. Example: TS-850 XVRT TX level is +10 dBm --> HF TX ATT value needs to be 20 dB.

 
How to set TX chain levels:

• optimal output power of 25 W Javornik version is 5 ... 25 W, and 5 ... 50 W for 60 W version; higher IMD levels (splatters) are created by pushing it to higher output powers; higher TX noise is produced when transmitting with low output powers. In case PA that follows XVRT needs less than 5 W of drive level use power attenuator at the XVRT output to reduce the power.
• fully open PIN attenuator 
• set PA output power with potentiometer in the XVRT (i.e. 500 W)
• use PIN attenuator to reduce the level by 1 dB (i.e. from 500 W to 400 W)
• set the output power to the previous level with potentiometer in the XVRT (i.e. back to 500 W)
• PIN attenuator has now very fine output power control; it can be increased by 1 dB (+25%) or reduced by about 8 dB (6 times).