The signal from the antenna passes the RF BPF or LPF (as a receive LPF, it divides the frequency band of 30 kHz to 60 MHz into 12 ranges) and RF Amp (or bypasses it) to be sent into the first mixer. Because in the first mixer section, a different mixer is used for the up conversion and down conversion respectively, the suitable mixer is selected according to the conditions.
Figure 1-2 describes the circuit configuration around the first mixer of the down-conversion path, showing the relationships between frequencies upon receipt of a 14 MHz signal.
The receiver mixer circuit is a quad mixer consisting of four 2SK1740 JFETs.
The mixer circuit achieves superior characteristics thanks to the revision of I/O port matching and the optimization of biases.
With the signal provided by the first local oscillator, the RX signal is converted to 11.374 MHz (first IF frequency).
The converted RX signal passes the first roofing filter of pass bandwidth 6 kHz and in the subsequent stage the signal is moderately amplified by the post amplifier, and sent into the second roofing filter. Part of the signal is also sent to the noise blanker.
The role of the first roofing filer is to limit the bandwidth for the sake of the noise blanker. We have selected a pass bandwidth of 6 kHz that does not affect pulse noise. Besides, by setting the intercept point of the post amplifier higher than that of the mixer, the deterioration of the two-tone characteristics is minimized within the pass bandwidth.
For second roofing filters, two 6-pole MCFs of 500 Hz and of 2.7 kHz respectively are equipped as standard at the time of purchase of your transceiver. Which filter is used is automatically determined according to the final pass bandwidth, i.e. depending on the conditions including the bandwidth selection made with WIDTH or LO CUT/ HI CUT controls on the front panel.
For example, in CW or FSK mode, if WIDTH is 500 Hz or less, the 500 Hz filter is selected and if WIDTH is 600 Hz or more, 2.7 kHz filter is selected. In SSB mode, if the difference between the HI CUT and LO CUT frequencies is 2.7 kHz or less, the 2.7 kHz filter is selected and if the combination produces exceeds a difference of 2.7 kHz, the up-conversion path is automatically applied. (In SSB-DATA mode, if WIDTH is 500 Hz or less, the 500 Hz filter is selected.)
In AM and FM modes, because the pass bandwidth of the down conversion path is too narrow, the signal is received with the up conversion path.
These operations are used in the amateur radio bands of 1.8 MHz, 3.5 MHz, 7 MHz, 14 MHz and 21 MHz, and for other amateur radio bands including WRC bands, and for other frequency ranges of general coverage receiving, up conversion is used regardless of the mode and pass bandwidth. (Since this switchover is determined by the CPU taking various conditions into its criteria, the conversion path cannot manually be selected.)
Figure 1-4 is an image of MCFs. From left to right, there is the 500 Hz filter at 11.374 MHz that is used in down conversion and next is the 2.7 kHz filter at 11.374 MHz.
At the rightmost filter is the 2.7 kHz filter at 10.695 MHz that is used during the up-conversion.
Hints and Tips "Which type of conversion is used?"
• During the transmission:
The up-conversion configuration is always used in all modes and bandwidths. During the transmission in SSB mode, the pass bandwidth is determined by the filter settings (digital filter of the DSP) selected in the menu mode. The pass bandwidth of the filter in the analog stage is 6 kHz and does not affect the final outcome of the frequency analysis.
• During the reception in AM or FM mode:
The up-conversion configuration is always used regardless of the frequency or pass bandwidth settings.
• If WIDTH is switched from 500 Hz to 600 Hz during the reception in the 3.5 MHz band in CW mode:
While the down conversion configuration is maintained, the roofing filter is switched from 500 Hz to 2.7 kHz.
• LO CUT is changed to 200 Hz when receiving in the 14 MHz band in SSB mode with LO CUT 300 Hz and HI CUT 3000 Hz:
Because the final pass bandwidth exceeds 2.7 kHz, the operation is switched from down-conversion to up-conversion configuration.
• During the reception in the 50 MHz band in SSB mode with LO CUT 300 Hz and HI CUT 2700 Hz:
The up-conversion configuration is used. Though the pass bandwidth of the roofing filter is 15 kHz, the 2.7 kHz filter is selected at the second IF of 10.695 MHz.
Following is a graph that provides the comparison between the performances of roofing filters.
Figure 1-5 Comparison of Bandpass Characteristics of MCFs
Figure 1-5 compares the band pass characteristics of a roofing filter of center frequency 73 MHz (gray line); and the roofing filters of the center frequency 11.374 MHz with bandwidth of 500 Hz (blue line) and with bandwidth of 2.7 kHz (orange line) that are both employed by the TS-590S.
Because the center frequency of the filters differ, graphs are overlapped at the center frequency. The frequency indicated as 0 kHz at the center of the Frequency [kHz] axis is the receive frequency.
It is apparent that when down conversion is active, large attenuation is achieved at frequencies other than the target signal.
Figure 1-6 Comparison of Dynamic Range Characteristics
Figure 1-6 shows a graph comparing the dynamic range characteristics of TS-590S and TS-480S (with CW filter) that are measured by changing the frequency spacing with the interfering signal.
The abscissa axis shows the distance from the interfering signal. For example, it represents that at the point of 10 kHz the receive frequency is 14.200 MHz and two interfering signals of 14.210 MHz and 14.220 MHz are given.
The orange line shows the result of TS-590S and the gray line shows the result of TS-480S.
I n the event the frequency separation is greater than 20 kHz, the dynamic ranges of both transceivers exceed 105 dB; however, as the separation becomes smaller (the interfering signals come closer to the receive frequency), the dynamic range of TS-480S with conventional MCFs is decreasing. As the graph of pass bandwidth shows, this results due to the difference of attenuation at the roofing filter.
Note: In the receive frequency and its adjacent band, the measurement at the level of "3 dB higher than the ordinary noise level" may not be feasible due to influence of the noise generated from its local oscillator. Instead, the level, which has been reached to S5 with the measurement by an S-meter, is predetermined as the reference level, and the level is converted to the same level as the predetermined level, namely "3 dB higher than the ordinary noise level", and then appears on the graph of pass bandwidth. For comparison, both transceivers were measured using the same measuring method. The outcome is an example and does not warrant the performance of the product.