A 70cm Wide-band Transceiver Concept

This article provides a detailed overview of a 70 cm wideband transceiver designed for duplex operation. The modules operate within a 200 kHz wide duplex channel, selectable from 430-440 MHz with a 100 kHz channel spacing. Initial prototypes were introduced at the international Packet Radio Symposium in April 1997, with subsequent minor modifications based on further experience. Narrow-band packet radio access frequencies, utilizing 25 kHz channel spacing for either 1.2 kBaud AFSK or 9.6 kBaud FSK, have been standard for over 15 years across many regions in Europe. These transmission speeds are adequate for connecting to local BBS systems, participating in packet radio conversation groups, or accessing DX clusters. However, there is a growing demand for higher transmission speeds, approximately ten times greater than the standard 9.6 kBaud operation. Emerging applications include new BBS systems with HTML graphical user interfaces, digital voice transmission, and real-time image transmission in accordance with the ISDN standard. Inter-node links to support these requirements are currently being developed, but high-speed user access to the packet radio network remains limited. In Germany, a single 70 cm packet radio channel has been allocated for user access with a bandwidth of 200 kHz, which is currently under experimental status, and further channel assignments are uncertain. Consequently, the user transceiver is designed as a single-channel version without a user interface for frequency changes, although frequency modification through reprogramming is possible. The transceiver must be versatile enough to function as either a digipeater or a user station. To leverage high-speed operation, the transmit/receive delay should be less than 1 ms, and the signal-to-noise ratio (S/N) at the receiver must be 10 dB higher due to the wider bandwidth, matching the access area requirements of 9.6 kBaud operation. This enhancement can be achieved by increasing the transmit power by 10 dB or by utilizing an appropriate antenna gain. For user operations in less favorable locations and for digipeaters, an output power of 20 Watts is recommended, although lower output power may suffice based on experience. An alternative power amplifier design is proposed, offering an output power range of 2-7 Watts. A transceiver concept that meets these specifications was previously presented. Initial testing of prototypes revealed issues with parts availability and costs, leading to minor design adjustments. The transmitter's FSK signal is produced by modulating a voltage-controlled oscillator (VCO), which is stabilized via a slow phase-locked loop (PLL) to a specific frequency. The PLL loop filter has a cutoff frequency of 40 Hz, resulting in a minimum bit rate of 38.4 kBaud. This slow PLL is capable of compensating for long-term drift caused by temperature variations and aging, but rapid reactions during TX and RX switching must be avoided. Consequently, several buffer stages were incorporated between the oscillator and the first switching stage. Additionally, the power supply for each module requires effective decoupling. This is achieved by compartmentalizing the design into three separate modules: receiver, transmitter, and power amplifier, each housed in distinct enclosures. Only the power amplifier is switched between transmit and receive modes. The receiver, transmitter, and 7 Watt power amplifier are contained within tinned steel boxes measuring 74 x 111 x 35 mm, while the 20 Watt power amplifier is housed in a cast aluminum enclosure with an attached heat sink, measuring 120 x 95 x 34 mm. The transmitter employs a VCO operating at the output frequency, utilizing a BF 979 transistor and a 20 mm length of semi-rigid cable as the oscillator inductance. The VCO is stabilized using the Siemens PLL circuit SDA 3302-5.

The design of the 70 cm wideband transceiver emphasizes modularity, allowing for ease of maintenance and upgrades. Each module can be tested independently, which simplifies troubleshooting and enhances reliability. The use of a slow PLL for frequency stabilization is particularly advantageous in maintaining signal integrity over extended periods, ensuring that the transceiver can operate effectively in varying environmental conditions. The decision to implement buffer stages is crucial for preventing signal degradation during the transition between transmit and receive modes, thereby enhancing overall performance.

The choice of materials for the enclosures, such as tinned steel for the receiver and transmitter and cast aluminum for the power amplifier, is significant for thermal management and electromagnetic shielding. This helps to minimize interference and maintain signal clarity, which is critical in high-frequency applications. The modular design also allows for the potential integration of additional features in the future, such as advanced digital signal processing capabilities or enhanced user interfaces, should the demand for such functionalities arise.

In summary, this transceiver design is a comprehensive solution for modern packet radio applications, accommodating both current needs and future advancements in technology. The adaptability of the transceiver to function in various roles within the packet radio network, combined with its robust design and performance specifications, positions it as a valuable asset for users seeking reliable high-speed communication in the 70 cm band.This article presents a description of a 70cm wideband transceiver for duplex operation. The modules are designed for operation in a 200 kHz wide duplex channel, freely selectable from 430-440 MHz with a 100 kHz channel spacing. First prototypes were introduced on the international Packet Radio Symposium in April 1997. Some minor changes resulting from further experience were added to the text since then. Narrow-band packet radio net access frequencies with 25kHz channel spacing for operation with either 1k2 Baud AFSK or 9k6 Baud FSK are state-of-the-art and standard for over 15 years now. They are common in wide areas of Europe. These transmission speeds are sufficient for connecting local BBS systems, taking part in packet radio conversation groups or accessing DX clusters.

There are, however, new emerging applications requiring higher transmission speeds. These requirements ask for higher speeds of one order of magnitude compared to the standard 9k6 Baud operation. Examples for such applications are new BBS systes with HTML graphical user interfaces, digital voice transmission or image transmission in real-time following the ISDN standard.

Inter-node links following those requirements are currently being set up. High-speed user accesses to the packet radio network are still missing. In Germany, there is currently a single 70cm packet radio channel assigned for user accesses with a bandwidth of 200kHz. This channel has experimental status and the assignment of further channels cannot be foreseen. Therefore, the user TRX is designed as a single channel version without any user interface for changing the frequency.

The frequency however, should be changeable through re-programming. The transceiver must be adaptable to be used either as part of a digipeater or a user station. To utilise the benefits of high-speed operation, the transmit/receive delay should be smaller than 1 ms. The S/N at the receiver has to be 10 dB higher due to a wider bandwidth, if the access area requirements are similar to 9k6 operation.

This can be achieved by either increasing the transmit power by 10dB or the utilisation of an appropriate antenna gain. For user operation in unfavourable locations and on digipeaters, the authors recommend an output power of 20 Watts.

From experience, lower output power may be sufficient. Therefore, an alternative power amplifier is presented with an output power of 2-7 Watts. A transceiver concept was presented in [2] that meets the above requirements. First experience with prototypes and problems with parts availability and parts cost led to some minor design changes. The transmitter FSK signal is generated by modulating a VCO which is stabilised through a slow PLL to a frequency.

The loop filter of the PLL has a cut-off frequency of 40Hz, resulting in a minimum bit rate of 38k4 Baud. This slow PLL can only compensate for long-term drift caused by temperature- and ageing effects. Reactions, resulting from switching between TX and RX, have to be avoided. Therefore, a few buffer stages were added between the oscillator and the first switched stage. Also, the power supply of the individual stages has to be well decoupled. This can only be achieved with moderate efforts if the concept is divided in three separate modules: receiver, transmitter and power amplifier, all in separate boxes.

Only the power amplifier is switched between transmit and receive phases. Figure 1 shows the three blocks. Receiver, transmitter and the 7 Watts power amplifier are accommodated in tinned steel boxes of size 74 x 111 x 35 mm. The 20 Watts power amplifier is accommodated in a cast aluminium box with an attached heat sink, 120 x 95 x 34 mm in size.

The transmitter uses similar as in [1] a VCO operating on the output frequency. It uses a BF 979 transistor and a 20mm piece of semi-rigid cable as the oscillator inductance. The VCO is stabilised by the Siemens PLL circuit SDA 3302-5 🔗 External reference