YAESU FT 950 SSB 2,4Khz roofing filter

As a backup to my FT 2000 radio I am using a FT 950, which has the same 69,450MHz first IF frequency. The  first IF stage features three roofing filters (15 kHz, 6 kHz, and 3 kHz) automatically selected by mode. Each roofing filter is a four-pole, fundamental-mode monolithic crystal filter design. Unfortunately, some choices made in the first IF design of the rig left is with quite a poor performance score in one area - close-in high-strength signal handling capability. The original Yaesu 3 kHz filter measures only about 7 kHz wide at -6dB.  After Jeff AC0C wrote that  FT2K/FT950 excellent NS roofing filter in no longer in a production I have decided to make my own roofing filter. Roofing filter attenuates and reduce IMD3 products and DSP has less bandwith to process.

After I successfully installed in the second IF 450kHZ a selection of  2 (two)  Murata +/- 2kHz CFWLA450KJFA ceramic filters placed in a cascade and  therefore reduced IF noise and IF image(see picture 1) ,  I also decided  to improve the FT 950  input IMD performance with my own SSB roofing filter.  The same roofing filter can also be used for FT 2000 receiver.

Picture :1  New 450Khz  IF

A homemade SSB the 4 pole 50 Ohm Cohn crystal filters of 2,7 KHz bandwidth at  6dB has been modeled with a AADE Filter design V4,5 program and is built from discrete 3rd overtone crystals. Crystal parameters:  3rd overtone  FL 69,450 Mhz , RR = 20 oHm, C0 = 4pF, FL= 5ppm, package UM1  (see picture 2 and 3)

Picture:2 Cohn  4 pole filter  modeled with an AADE Filter design

Picture:3  Input impedance modeled with an AADE Filter design

To improve the impedance mismatch between 1st mixer and roofing filter I added a diplexer to properly terminate the roofing filter. Diplexer terminates the impedance of the roofing filter to the first  mixer in a wide frequency range. To compensate filter loss I have added ultra-low noise (0,55dB) and  high performance MMIC LNA- SPF 5189Z (or MMIC PGA 103+)with output third order intercept point typ +38.5 dBm. To lower the MMIC  gain a 4dB T attenuator has been built at the output.(see picture 4)

Picture: 4  The 69,450Mhz roofing filter circuit

Unlike Jeff AC0C, I decided to improve the ultimate attenuation and   installed my new 2,7 kHz roofing filter before the original YAESU 3kHz XF1004 filter.( see picture 5)

Picture: 5 

Before the 2,7 kHz roofing filter has been built in the Yaesu 3 kHz filter measures about 6-7 kHz wide at -6dB. The spectrum is taken from the 2nd IF TP 1056 and shows  the filter shape of the original FT filter.( see picture 6)

Picture 6 : Spectrum taken before 2,7kHz  roofing filter has been built

Preparing the 2,7 kHz roofing filter for installation

1.       Locate the working area on the PC board ( see picture 7)

Picture 7: Working area

2.       Unsolder SMD capacitor C 1583 (see picture 8)

Picture: 8

3.  Cut the PCB trace between T1026 and original Yaesu XF 1004 3kHz filter (see picture 8)

4. Solder the input coax center conductor to the center pads of T1026 (see picture 8)

5. Solder the input coax shield to the transformer T1026 casing.(see picture 8,9)

Picture: 9

6. Solder the output coax center conductor to the input pad of original XF1004   Yaesu 3 kHz filter (see pictures 7,8,9)

7. Solder the output coax shield to the ground near filter XF1004 (see pictures 7,8,9)

8. Solder back SMD the capacitor C 1583

9. Connect 9V DC to the new filter from pin 14 - J 1005 ( see pictures 10,11,12)

Picture:10

Picture: 11 

Picture:12

9. Trim T1026 and T1033 to maximum signal level

After the 2,7 kHz roofing filter has been built in ,(see picture 13)  the front end is far more selective and a new filter measures about 2,7 kHz wide at -6dB and 8kHz at -50dB. The spectrum is taken again from the 2nd IF TP 1056 and it shows a new filter shape .(see picture 14)

Picture: 13 Roofing filter  build in

Picture :14  Spectrum taken after 2,7kHz roofing filter has been built in

More about the built in procedure can be seen from the pictures above.

After this modification and  adding a selection of  2 (two)  4kHz Ceramic filter in second IF 450kHz ,   I measured IMD and RMDR dynamic range at offsets of 2kHz  and 5kHz  with   2 x -6dBm  DC4KU   HF- Zweitongenerator FA-2-HF.  Two-tone third order dynamic range (IMD DR) is the diference between MDS and the levels of two interfering signals causing IMD products 3dB over the noise floor. RMDR is Reciprocial mixing dinamic range , measured as a 3dB increase in noise floor.    See Picture 15 

Picture 15: DC4KU  Two- tone RF generator -6dBm

Results of two tone IMD testing - 1.st preamp OFF ; 500Hz Bandwith; Roofing 2,7KHz; AGC Off , Test band is 7MHz ;

MDS = - 128 dBm, 
Blocking above noise floor @ 10 kHz ,measured as 3dB increase in noise floor  107 dB
IMD2 Dinamic range second order on 14 Mhz    = 76 dB

Spacing offset:                                              IMD DR3:                    RMDR              

2  kHz                                                               86dB                       90 dB           
5 kHz                                                                88dB                       98 dB              

According to the experts’ opinion (e.g. Rob NC0B and Tom W8JI), the IMD DR3 of 85 dB is enough for CW. Thus my 2 kHz IMD DR3 86*dB results are very good.

I have been testing the new roofing in CQWW_2019 CW/LP   contest with more than 2010 QSOs  and I was really satisfied with a new installation. I used the radio on the air  several days before  CQ  WW Contest and great difference was noted  between the 6kHz filter and the new 2,7 kHz filter at dynamic range

With a little changes the same filter can be build also in FT2K

Warning !!! Changing any hardware inside FT950 will void your warranty.I am not responsible for any hardware failure. You made this mods on your own risk.! This modification requires a high level of soldering skill, possibly beyond that normally possessed by the average HAM. Professional assistance is advised if you are not confident that you have  this ability.

Note for simplified installation  10.08.2020

I found that is much easier  to  unsolder (zero) oHm  SMD resistor R 1393 to insert  roofing  filter in cascade with original Yaesu 3kHz roofing filter. See pictures  below.

FT 2000 Modification

FT 2000  Modification - Added SMD diplexer and reducing C1334

It is surprising that results of Rob Sherwood’s (NC0B) tests show that FT950 has 16dB better 2 kHZ Narrow Spaced Third Order Dynamic Range than its “bigger brother” FT2000.The problem has been identified by VE3GNO/YO3GJC and US6IQ some time ago. The first reason is poor impedance matching between first mixer SPM5001 and roofing filter. The second  reason is improper  first mixer loading  with a  10pF capacitor C1334 for a DMU Bandscope unit.  See  Picture 1.

Picture 1: FT 2000 Shematics



In the newer receiver FT 991 with the same first mixer SPM 5001Yaesu has improved this impedance mismatch and added a diplexer to properly terminate the roofing filter. Diplexer terminates the impedance of the roofing filter to the mixer in a wide frequency range. The Yaesu FT 991 has also the same first IF frequency of 69,450 MHz as FT 2000. See Picture 2

Picture 2:  FT 991 Diplexer

As an enthusiastic contester, I wanted a better receiver performance so I decided to insert a SMD diplexer at the output of SPM5001 mixer and T1021 transformer circuit. The added SMD diplexer was built into a housing of the first mixer. See Pictures 3 and 4.

Picture 3: Diplexer shematcs f= 69,450MHz

Picture  4: Location of  the  SMD diplexer 

Replacing the coupling capacitor for the bandscope.

As the original 10pF capacitor C 1334 inappropriately loads the first mixer, I reduced it to 1pF, which is enough to connect the RTL SDR  receiver for a panoramic adapter. When the SDR panoramic adapter or original Yaesu DMU Bandscope unit is not used, remove the capacitor C 1334.

Picture 5:   C 1334 location and  T1021 transformer output pin.

Picture 6:   C 1334  location and  T1021 output pin.

To reach the elements of the mixer,the mixer housing at the bottom side of the PC board must be unsoldered.At first unsolder mixer shield on bottom PC board side and take it out. see Picture 7

Picture 7:   Unsolder mixer shield

Preparing the diplexer for installation

1. Locate the work area on PC board. (see Picture 8)
2. Unsolder and take out mixer shield on bottom PC board.  (see Picture 7)
3. Unsolder SMD capacitor C 1334, 10 pF. (see picture 5 and 6)
4. On its place solder new SMD capacitor 1 pF.
5. At the existing original transformer   T1021 carefully unsolder the output (right) wire.  (see Picture 5 and 6 )
6. Solder the transformer T1021 output   wire  to the diplexer input.
7. Connect the diplexer output back to the place on the board where the transformer T1021 has been unsoldered.  
8. Solder mixer shield back.( see Picture 8) 

Picture 8: Final installation

Diplexer has an attenuation of 0.7 dB, but I did not detect any difference in signal strength after the diplexer has been built in. But for improving receiver,s dynamic range  is recommended to redistribute the RF gain reducing it ahead of the overloaded  stage ( 2nd mixer) and building it up again after the narrow IF filter. However we can change IF gain for 1dB in FT 2000  hidden  menu setting  B11 to B20  if  we want. But be very careful - this is a hidden service menu accessible for maintenance by a qualified technician only!

More about the build in procedure can be seen from the pictures.

After this modification and already some years ago built-in AC0C 2 kHz NS roofing filter, adding 4kHz Ceramic filter in second IF 450kHz ,   I measured IMD dynamic range at offsets of 2kHz , 5kHz and 20kHz  with   2 x -6dBm  DC4KU   HF- Zweitongenerator FA-2-HF.  Two-tone third order dynamic range (IMD DR) is the diference between MDS and the levels of two interfering signals causing IMD products 3dB over the noise floor.RMDR is Reciprocial mixing dinamic range,measured as a 3dB increase in noise floor.. Picture 9

                                 

Picture 9: Two tone RF generator 2x -6dBm

Results of two tone IMD testing - 1.st preamp Off; 500Hz Bandwith; Roofing AC0C 2,7KHz; Test band is 7MHz ;
MDS = - 138dBm, IMD2 Dinamic range second order on (14102kHz)=87dB , Blocking above
noise floor @ 10kHz ,measured as 3dB increase in noise floor =107dB

Spacing offset:            IMD DR3:           RMDR

2  kHz                           86dB                  90dB

According to the experts’ opinion (e.g. Rob NC0B and Tom W8JI), the IMD DR3 of 85 dB is enough for CW. Thus my 2 kHz IMD DR3 86dB and RMDR 90dB results are very good.. 

In the future I will still try  to improve  the receiver,s dynamic range  with option to increase the mixer Oscillator injection levels. The  simplest option is to increase R 1319 at first mixer SPM5001. We,ll see.....

Disclaimer

Warning!!! I am not responsible for any hardware failure. You make this mods at your own risk! This modification requires a high level of soldering skill, possibly beyond that normally possessed by an average HAM. Professional assistance is advised, if you are not confident that you have this ability.