1.1 Type of Conversion
Receive performance is one of the key indicators that is used to evaluate a transceiver. And, above all, the capability to protect against interference from adjacent signals close to the target signal is of the utmost importance.
To attain this goal, a circuit with a good large signal behavior characteristic is used for the first mixer of the RX section. In recent years, a filter used between the mixer and the subsequent stage (roofing filter) is also gaining much attention as a very important component.
About 30 years ago, an up-conversion circuit configuration (where the first IF is higher than the upper limit of RX frequency) appeared as an RX circuit design to provide general coverage receiving from LF through the HF band. This RX system was also adopted by amateur radio transceivers of the time to enable reception of overseas broadcasting and other signals outside amateur bands and, as a result, from that time on, almost all HF transceivers have been equipped with an up-conversion RX section.
The passband of roofing filters used in an up-conversion RX design is typically 15 to 20 kHz. However, in the case an interfering signal is only several kHz away from the target signal, the interfering signal also passes the roofing filter and the target signal is masked first in the subsequent stage. As a result, sometimes the performance of the first mixer was not extended to the best use.
That is the reason a design to switch the pass bandwidth to be 3 kHz, 6 kHz, or 15 kHz is becoming prevalent in recent transceivers. Some products can select a bandwidth as narrow as several hundred Hz and these products are very highly accepted in the market.
Meanwhile, Kenwood’s HF transceivers, which were designed 7 years ago or earlier, adopt roofing filters with a wide passband. Obviously, they still have satisfactory performance outside the pass bandwidth.
Against this backdrop, we started the development of the TS-590S by considering the circuit type that mostly focuses on the characteristics of adjacent interference elimination.
In the early stage of the TS-590S’s product development, considering the product positioning in the market, we also examined the RX design to be able to switch among the roofing filters of 3 kHz, 6 kHz and 15 kHz. However, the bandwidth of 3 kHz is too wide for CW, though it is fairly narrow for an SSB. We wanted to adopt a 500 Hz filter by all means for CW enthusiasts. However, there was a big challenge to be solved.
When it comes to the pass bandwidth of a roofing filter, at a frequency as high as 73 MHz, which is Kenwood’s mainstream first IF frequency, it is difficult to mass-produce filters with bandwidth as narrow as 500 Hz. To solve this problem, there was no other choice but to lower the first IF frequency.
After reviewing, we decided to lower the first IF to 11.374 MHz. This is called a down-conversion design. (If the receive frequency is lower than 11.374 MHz, the operation will be up conversion. However, because the first IF is lower than the highest receive frequency (60 MHz), we call the conversion type “down conversion”.)
Yet, this circuit design has a drawback. When the IF frequency that was once raised 30 years ago to provide general coverage reception is lowered again (to 8.83 MHz that was then used), images and spurious signals are produced (which are relevant not only to reception but to transmission) and these causes must be addressed one by one.
Needless to say, it is technically possible to tackle individual problems but, to do so, many additional circuits and components are required, which may result in a higher product price. In terms of market positioning, TS-590S must be a product in a competitive price range having higher cost-to- performance ratio. After examining various frequency configurations, we have selected a dual-mode conversion frequency configuration for the new TS-590S to satisfy both the performance and price requirements.
