How To simply improve FT 950 Receiver Dynamic Range.

After I measured the receiver noise level on my antennas and I measured level between -73dBm and -121dBm (S-9 to S-1) on 500Hz  CW  bandwith, I found out that the receiver is too sensitive on 160, 80 and also on 40m. 

Picture 1:  Noise level measured on my antennas.

Picture 2:  ITU Noise level guidance

The noise from antenna was measured  with a calibrated S meter.
The antennas used in the Noise level determination were as follows:

• 1,8 MHz   40m long Inverted L
• 3,5MHz    20m long Inverted L
•    7MHz    Dipole
• 14 MHz    3el.Beam
• 21 MHz    3el.Beam
• 28 MHz    3el.Beam

Measurements were made at several times and with attempts to measure on the same frequences at similar day times.

How Much sensivity  Do We Need ? Arbitrary assumption are made: Add to measured receiver noise level 15dB. Oversensivity makes no sense ,only HF receiver Dynamic Range gets worse.

On the bands from 160 to 40m you should not use a preamp att all, because the antenna noise at these frequencies is still above the noise floor.  In order to improve the dynamic range I always switch on at lower bands IPO and  6-18 dB  antenna attenuator, which significantly improves the received signal-to-noise ratio. Also note that IMD3 products drop x3 by the selected attenuator value. For example: 6dB attenuator increase  IMD 3  for 18 dB !!.

In the FT 950 receiver  I have also  reduced   IF gain for about 10 dB. ( see  FT 950 Tehnical Supplement IF Gain Adjustment- Page ALIGNMENT 7 )

Alignment Parameter E-01iGn , E-02iGn and  E-03iGn   represents the receiver's  IF gain for 160, 80 and 40m. Turning  CLAR (VFO B)  Knob clockwise  increase the value and   a higher number  will reduce the IF gain. On 160 ,80m and 40m  I increase the value by 10. One value equals approximately 1 dB. Please be carefully, because this Hidden service menu is accessible for maintenance for an tehnican experts. 

You made this mods on your own risk. Do any of the setting at your own risk  ! 

Keep away from any modifications  if you are not sure.  

This modification requires a high level of  skill, 

 possibly beyond that normally possessed by the average HAM. 

Professional assistance is advised if you are not confident that you have  this ability. 

I could not be responsible if you,re doing something wrong. 

FT 920, FT 897D and FT1000MP audio white noise reduction

 

The  FT 920, FT 897D and FT 1000MP has the same audio amplifier TDA2003, where  white noise was rather disturbing. Therefore I added a recommended feedback  according TDA 2003 data sheet description. According to the recommended  formula, the RC circuitry cuts tones higher than 3 kHz. The additional serial RC circuit can be directly soldered between pins 2 and 4 of theTDA2003. See picture 1

For  SSB ( 2,7kHz)     Rx= 39Ohm    Cx= 270nF

FT 950 narrow IF filter

I am using the FT 950 as a second rig as a backup for my FT 2000. The FT 950 use  the same 450 kHz Ceramic filter as the FT 2000 transceiver . The CF 1001  has been built in the second 450kHz  IF with a BW of about 15kHZ, because the FT 950 can be used in FM as well as in AM mode. 
see: Picture 1

Picture 1: FT 950 IF schematics.

However for SSB and CW the original filter CF1001 -CFWLB450KE1Y is too wide to reduce the signals that are reaching the DSP.( see Picture 2)

 Picture 2: Original IF (Measured on TP 1054, Roofing 15Khz).

Because I don't work on FM and AM mode I decided to install a selection of 2 (two) Murata +/- 2Khz CFWLA450KJFA ceramic filters placed in a cascade.

Picture 3: Murata +/- 2Khz filters placed in a cascade.

Cascaded Murata CFWLA450KJFA filter will reduce much better stopband suppression and bandwidth on SSB and CW is reduced to 4Khz. Therefore it will also reduce IF noise and IF image. FM will be degraded, but AM not excessively.  Normal 2,4 kHz SSB and CW will not be affected. After these changes the signals will be better managed with a DSP.

Picture 4: New bandwith (Measured on TP 1054, Roofing 15Khz).




Preparing the new Cascaded Murata CFWLA450KJFA 4kHz filter for installation:


1. Locate the work area on the PC board (see picture 5)

Picture 5 :  Work area location.

     2. Unsolder SMD resistance R 1415, 0 Ohm (see picture 6)
3. Unsolder SMD resistance R 1433, 390 Ohm (see picture 6)

Picture 6 : 

4.   Stick with glue 2x new  4kHz Murata filters on the existing original  filter CF1001 (see     picture 7).

5.   Connect with a thin  wire the output from a transformer T1027 with the first  new 4Khz     Murata  filter input.

6.   Connect with a thin wire the output from first new 4Khz Murata filter with an input of the     second 4Khz Murata filter.

7.   Solder back with a thin wire the 390 Ohm SMD resistor between the second  Murata  4kHz   filter output and T 1030 input.

8.   Adjust carefully transformers T1027 and T 1030 for maximum S meter deflection 
 (see picture 8).

Picture 7: The final filter installation.

Picture 8 : 450kHz  filter layout and  T1027 and T1030 transformer location.

One Murata CFWLA450KJFA filter has 6dB attenuation, but I did not detect a large difference in signal strenght after the new filter, which has been built in. 
Because the FT 950 receiver is too sensitive on low bands, I decided, after this modification not to increase an IF gain.  However, we can change the IF gain in the FT 950 Hidden Menu setting E01iGN to E10iGn if we want.  (Please see FT 950 Technical Supplement, page Alignment –IF Gain adjustment.)

Please carefully note that this  Hidden service menu is accessible for maintenance for an technician experts. More about the build in procedure can be seen from the pictures.

After building new filters it is still possible to receive AM, but FM will be degraded. I have been testing my receiver in CQWW_2019CW contest with more than 2000 QSOs and I was really satisfied with the new installation.

CFWLA450KJFA filter can be purchased online in Poland at: https://www.hesta.pl/pl/

Picture 9 : Murata  4Khz filter specifications.

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 an average HAM. Professional assistance is advised if you are not sure if you have this ability.