Effect of tuning on the sideband noise levels in a FT-736R

A comparison between figures 1 and 2 shows that the sideband noise of the FT-736R is degraded when the output power is reduced to the level normally used to drive a power amplifier at the SK4BX club station. These spectra show the performance when the spurs due to the sidetone have been eliminated by a filter on VCC for Q26 and Q27.

Fig.1 Carrier spectrum of the FT-736R at full power.

Fig.2 Carrier spectrum at reduced power (-4.6dB).

It is obvious that a reduced level for the carrier (by 4.6 dB) causes an increase of the noise floor in absulute terms. The sideband noise in dBc/Hz is degraded by about 10dB at a frequency separation of 10 kHz.

It is not uncommon that the noise floor stays at a constant level when the level of the carrier is turned down on a modern transceiver. In such cases the sideband noise typically originates in the low level transmit amplifier immediately after the crystal filter. An increased level as observed in the FT-736R is created by AM modulation somewhere. The amplifier Q23 is fed directly from 8V and this amplifier stage is probably more sensitive to noise on the 8V supply line when the power potentiometer is turned down causing a smaller current through this transistor.

We did not have the time to do a careful investigation at the Annaboda meeting, but we found a quick fix to the problem in mis-tuning TC6001 and TC6002, the output of the driver amplifier. By tuning these capacitor for only a marginal ALC action (S3 on the meter) the influence of the ALC circuit was reduced significantly. The loss of gain caused the output power to fall by about 1 dB but since this particular rig is typically used at a somewhat reduced power level to drive a large power amplifier a slightly reduced maximum power output is perfectly OK. By mis-adjusting some other ALC adjustment it should be possible to get full power at a very small ALC action.

Figures 3 and 4 show the spectrum of a carrier at full power (1 dB below original full power) and at the power level used to drive the big power amplifier. Table 1 shows the sideband noise in dBc/Hz as extracted from figures 1 to 4.

Fig.3 Carrier spectrum at full power with TC6001 and TC6002 mistuned for ALC indicatinf S3 on the meter.

Fig.4 Carrier spectrum at reduced power with TC6001 and TC6002 adjusted as in figure 3.
                           Noise floor in -dBc/Hz  
                        5kHz  10kHz  15kHz  20kHz  50kHz
FT-736R (3J740239)     116.6  124.4  126.4  127.5  129.4
FT-736R mod1           117.2  124.4  126.7  127.6  129.9
FT-736R mod1,lowpow    112.5  113.8  115.3  116.3  123.8
FT-736R mod2           117.2  124.1  127.0  128.1  129.9
FT-736R mod2,lowpow    115.9  119.1  121.1  122.1  128.5

Table 1. Noise floor at different frequency separations from a carrier.