FEF-CMMHQCCBA&T (Front end filter comutable multi-monocrystal for heavy qrm conditions, connected between antenna and transceiver)

    Many times, during rtty contests, I have a big problem with qrm. A lot of strong signals in a small bandpass, so, if you reduce the RF Rx gain or connect the 6-12 dB attenuator you shall loose the weak signals. In fact, the receiver (the preamplifier, mixer, etc) have to deal with a lot of unwanted (by the operator) signals, so the transceiver performance will be degradaded. Usualy, the modern transceiver have  a quality FEF (Front End Filter) after the 1'st mixer. It's wonderfull, but still need to reduce the RF Rx gaind during some periods of the contests.

So, last cqwwrttycontest  (2020) I tried an old solution, a FEF connected direct between antenna and transceiver, with 5 comutabloe monocrystals (7041, 7045, 7055, 7070 and 7082 kHz.).

1'st, some history....

Few explanations: circuits A & B give us the posibility to have a small variable bandwidth; circuit C is a simple one, with the minimum bandwidth obtainable from a specific crystal. A short theory lesson for the "C" configuration we can find at page 106 of The Radio Amateurs Handbook - 14th edition.

And now, others people experience:

Eddystone "experience"

(http://eddystoneusergroup.org.uk/Restoration%20projects/Tech%20Short%205%20-%20Receiver%20Selectivity%20and%20Crystal%20Filters.pdf)

and M0AYF QRSS band pass filter

And now, my experience:

Note 1.

Even you know the Fs of a crystal can be modified with a C in series, you shall find that the RL, or selectivity is depending how far you are from Fs. So, for a crystal with Fs= 7045 khz, the optimal aria for this kind of filter is very close from Fs, only 1 khz up and down (even you can move Fs with 5-6 khz up, with Cv=5-10 pF). The explanation is Q factor wich is degradeding when you modify Fs.

Note 2.

Ct have to be a good trimer, air or mica, with small capacity. Dont forget that you have to adjust somewhere around 3-5 pF, exact the value of  crystal Cp.

Note 3.

Find a good variable rezistor, with a very low rezidual inductance.

General note

Take care of practical setup: minimum wires, short wires, screening. If you can not adjust Ct for a good diagram, it seems you have parasitic capacitance in circuit, wich are greater than your Ct. Also, dont forget you need another module, the relais module. You need to disconect the FEF during Tx, to bypass the FEF and you are not using him, etc. You need minimum 3 good relais: the antenna connector, the transceiver connector and the relais for commanding those 2 relais. The circuit depends what transceiver you are using, etc.

How to adjust the FEF (Front End Filter)

My first prototype: only 2 crystals, and a toroidal transformer on the output, to transceiver.

The adjustment is done with a RigExpert analyzer (AA-230 Zoom): 1st step adjusting the SWR for the lowest value (in the bandpass), and after that checking the whole diagram for RL (returning loss). If the value is more than 30 db, is OK (dont forget, 1 S point = 6dB, so 30 dB = 5 S points attenuation for unwanted signals).

Actual prototype has 5 crystals, a plastic box with aluminium tape for screening and a variable rezistor replacing the second toroidal transformer. It was not my best day when I put everything inside.... but I promise (to me) to build a more adequate aluminium box for cqwpxrtty contest.

Dont forget to adjust also the rezistor for a minimum SWR. Depending your crystals and setup you can obtain RL up to 50 dB.

... hmmm... I am litle shame of my presentation, I dont know if you understand what is in last 2 pictures, but the device is working.

How to use ? Very simple. Connect the device inline, put on BYPASS, and when have the biggest QRM around a frequency where you have crystals inside, connect the unit and try the Cv for an optimum Rx. Also, dont forget the RX RF gain, because nothing is an ideal solution. 

    So, thank you for your attention, and do not hesitate to ask if you need more info.

Good luck in contest (and QRM), 

YO4DFT - Cristian

cristian.moldovanu@gmail.com

P.S.

2 crops from my log, when I used FEF to deal with QRM.

..................

QSO:  7071 RY 2020-09-26 1530 YO4DFT        599 20 DX  RV3ZN         599 16 DX    

QSO:  7070 RY 2020-09-26 1533 YO4DFT        599 20 DX  LY3CY         599 15 DX    

QSO:  7069 RY 2020-09-26 1537 YO4DFT        599 20 DX  JH4UTP        599 25 DX    

QSO:  7069 RY 2020-09-26 1538 YO4DFT        599 20 DX  S57X          599 15 DX    

QSO:  7069 RY 2020-09-26 1540 YO4DFT        599 20 DX  SP2UU         599 15 DX    

QSO:  7069 RY 2020-09-26 1541 YO4DFT        599 20 DX  YL2BR         599 15 DX    

QSO:  7069 RY 2020-09-26 1541 YO4DFT        599 20 DX  UY7MM         599 16 DX    

QSO:  7069 RY 2020-09-26 1542 YO4DFT        599 20 DX  DA2X          599 14 DX    

QSO:  7069 RY 2020-09-26 1543 YO4DFT        599 20 DX  LY9Y          599 15 DX    

QSO:  7069 RY 2020-09-26 1543 YO4DFT        599 20 DX  RA3TT         599 16 DX    

QSO:  7069 RY 2020-09-26 1543 YO4DFT        599 20 DX  YL3CU         599 15 DX    

QSO:  7069 RY 2020-09-26 1544 YO4DFT        599 20 DX  SM5MX         599 14 DX    

QSO:  7069 RY 2020-09-26 1545 YO4DFT        599 20 DX  OK5NW         599 15 DX    

QSO:  7069 RY 2020-09-26 1548 YO4DFT        599 20 DX  IT9RBW        599 15 DX    

QSO:  7068 RY 2020-09-26 1549 YO4DFT        599 20 DX  IZ3BUR        599 15 DX    

.......................

QSO:  7040 RY 2020-09-26 2144 YO4DFT        599 20 DX  DH8BQA        599 14 DX    

QSO:  7040 RY 2020-09-26 2145 YO4DFT        599 20 DX  LA2XNA        599 14 DX    

QSO:  7040 RY 2020-09-26 2147 YO4DFT        599 20 DX  OH1SIC        599 15 DX    

QSO:  7040 RY 2020-09-26 2148 YO4DFT        599 20 DX  PA3BFH        599 14 DX    

QSO:  7040 RY 2020-09-26 2148 YO4DFT        599 20 DX  PB7Z          599 14 DX    

QSO:  7040 RY 2020-09-26 2149 YO4DFT        599 20 DX  DF2RG         599 14 DX    

QSO:  7040 RY 2020-09-26 2150 YO4DFT        599 20 DX  IU2LTO        599 15 DX    

QSO:  7040 RY 2020-09-26 2151 YO4DFT        599 20 DX  DJ1OJ         599 14 DX    

QSO:  7040 RY 2020-09-26 2151 YO4DFT        599 20 DX  UV5EDW        599 16 DX    

QSO:  7040 RY 2020-09-26 2152 YO4DFT        599 20 DX  IU4HRJ        599 15 DX    

QSO:  7040 RY 2020-09-26 2153 YO4DFT        599 20 DX  DL3SYA        599 14 DX    

QSO:  7040 RY 2020-09-26 2155 YO4DFT        599 20 DX  DL5ARM        599 14 DX    

QSO:  7040 RY 2020-09-26 2155 YO4DFT        599 20 DX  OZ1OXQ        599 14 DX    

QSO:  7040 RY 2020-09-26 2156 YO4DFT        599 20 DX  DM1FS         599 14 DX    

QSO:  7040 RY 2020-09-26 2156 YO4DFT        599 20 DX  R6CC          599 16 DX    

QSO:  7040 RY 2020-09-26 2157 YO4DFT        599 20 DX  DL3KUD        599 14 DX    

QSO:  7040 RY 2020-09-26 2157 YO4DFT        599 20 DX  DL4ME         599 14 DX    

QSO:  7040 RY 2020-09-26 2158 YO4DFT        599 20 DX  PA1JM         599 14 DX    

QSO:  7040 RY 2020-09-26 2200 YO4DFT        599 20 DX  DL0BI         599 14 DX    

QSO:  7040 RY 2020-09-26 2201 YO4DFT        599 20 DX  IU2CIQ        599 15 DX    

QSO:  7040 RY 2020-09-26 2204 YO4DFT        599 20 DX  IK1BPL        599 15 DX    

QSO:  7040 RY 2020-09-26 2206 YO4DFT        599 20 DX  SQ3MZ         599 15 DX    

QSO:  7040 RY 2020-09-26 2206 YO4DFT        599 20 DX  DQ9Y          599 14 DX    

QSO:  7040 RY 2020-09-26 2208 YO4DFT        599 20 DX  DL6UAA        599 14 DX    

......................

 

FT-920 + SCU-17 connections and settings

Yaesu FT-920 connexions with Yaesu SCU-17 for AFSK mode / RTTY, WSPR, FT4/FT8, etc.

Pin 1 (AFSK Input): Connect this pin to your TNC’s “AFSK Out” or “Mic Audio” output line. The optimum input level is 30mV rms, and the input impedance is 3 k. Your TNC’s audio output level potentiometer will allow you to set the level to the optimum value. This pin may be used either for 300 baud SSB-mode digital operation or for 1200-baud FM packet. The bandwidth and frequency response are not, however, suitable for 9600 baud operation. Be sure the AFSK-FSK switch on the rear panel is in the AFSK position. For FSK operation (whereby the TNC closes a line to ground to accomplish teletype keying), set the AFSK-FSK switch to FSK, and connect your

TNC’s FSK keying line to Pin 1.

Pin 2 (Ground): Connect this to the shield(s) of the cable(s) used for connections between the TNC and the FT-920.

Pin 3 (PTT): Connect this pin to the PTT line from the TNC. This pin, when grounded by the TNC, places the FT-920 into the Transmit condition.

Pin 4 (Audio Out): Connect this pin to your TNC’s “RX Audio” input line. This is a constant-level (100 mV rms @ 600) audio output line which is not affected by the position of the front-panel AF GAIN control.

Pin 5 (BUSY): This is a “Squelch Status” pin not generally required for digital mode operation. This pin is held at +5V when the squelch is open, and is grounded when the receiver is muted by the squelch (“nosignal” condition).

As viewed from rear panel (not from cable!)

Connector view

Data IN – 100 mVrms / 10k Ohms

Data OUT – 100 mVrms / 600 Ohms

FSK OUT – 25 Vcc/50mA/open colector

Data PTT - 25 Vcc/50mA/open colector

AFSK mode
FT-920 pin 1 Data IN (Audio IN)	SCU-17 pin 1 Data OUT (Audio OUT)
FT-920 pin 2 GND	SCU-17 pin 2 GND
FT-920 pin 3 PTT	SCU-17 pin 3 PTT
FT-920 pin 4 Data OUT (Audio OUT)	SCU-17 pin 5 Data IN (Audio IN)
FT-920 pin 5 Busy	SCU-17 pin 6 SQL IN (Busy)

Menu #	Description	Available choices	AFSK – RTTY	AFSK – WSPR	AFSK – FT4/FT8
U-42	RTTY Generator Tone Pair	Hi 2125U / Lo 1275U / Hi 2125L / Lo 1275L	HI 2125L	HI 2125	HI 2125L
U-43	RTTY Shift	170/425/850 Hz	170	170	170
U-44	Shift Polarity	Normal /Reverse	REV	REV	REV
U-45	Packet Freq. Display Offset	-3.0 ~ + 3.0 kHz	2125	0000	0000
U-46	Packet Subcarrier Frequenc y	1170/1700/2125/2210 USB/LSB	2125U	2125U / 1700U	2125U