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)
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
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
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
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.
