AOR_HF-7070_review.pdf

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1、 QEX July/August 2013 37 Colin Horrabin, G3SBI 17 Denbury Ave, Warrington, Stockton Heath, WA4 2BL, United Kingdom; The HF7070 HF/LF Communications Receiver Prototype A detailed look at high performance receiver design. The HF7070 receiver is a double con- version superheterodyne with a first IF at

2、45 MHz and a second IF that is centered at 44 kHz before going to a 25 bit audio ADC. The output from the ADC goes to an advanced 24 bit fixed-point DSP system. The radio covers dc to 30 MHz and has a noise figure of 12 dB without the use of a preamplifier before the first mixer. Out-of- band IP3 at

3、 50 kHz spacing is 45 dBm, giv- ing an SSB IP3 dynamic range of 115 dB. The IP3 within the 15 kHz bandwidth of the roofing filters is 19 dBm at 100 Hz spacing in an SSB bandwidth, which results in an IP3 dynamic range of 97 dB. This in-band linearity sets new standards for an up-con- version radio a

4、nd gives superb high fidelity reception of FM, AM, SSB and CW signals. To complement its excellent technical performance there are all the usual DSP fea- tures for the user. These include a sensitive band scope on the LCD panel of the radio. The band scope can also be displayed on a computer connect

5、ed via a TOSLINK opti- cal cable. With a noise floor of 145 dBm in a 50 Hz bandwidth, it can display sub- microvolt signals. The receiver analog front end has two H-Mode mixers using fast bus switches. The first mixer is terminated by quadrature hybrid-connected two-pole 45 MHz filters of 15 kHz ban

6、dwidth, which is followed by the first IF amplifier. This amplifier, with a noise figure of only 1.3 dB and an IP3 at 40 dBm, drives 4 poles of roofing filter, which is fol- lowed by a second amplifier. This amplifier drives the second H-Mode mixer that gives an output centered on 44 kHz. There is a

7、 balanced stage of amplification at 44 kHz before the 25 bit audio ADC. The 6 poles of roofing filter at 45 MHz gives 115 dB image rejection at the second H-Mode mixer, so an image rejection mixer is not required. The HF7070 was designed by the British electronic engineer John Thorpe of JTdesign bas

8、ed in Matlock, England. John also designed the Lowe receivers and the highly acclaimed AR7030 HF/LF receiver manu- factured by AOR UK. In terms of its technical performance the AR7030 represented very good value for money and was made from 1996 until pro- duction ceased in 2007 due to the restrictio

9、n of hazardous substance (ROHS) directive. Over that period some 5000 units were sold, most of which went for export (even the French Navy bought a few). Originally, John designed the HF7070 receiver for AOR UK, but they ceased trading two years ago. John is a consultant and had other design commitm

10、ents. So, I designed and built an up-conversion front end for the 7070 a few years ago to help with the development work. John was able to re-engineer this for mainly surface mount components and improve on its technical performance. Just after John was presented with the front end board, I got to k

11、now Martein Bakker, PA3AKE. Martein was keen to build a holy grail version of the CDG2000 transceiver and had made technical measure- ments on transformers and fast bus switches for H-Mode mixers. These measurements proved quite useful to John. Those read- ers who are familiar with the CDG2000 Amate

12、ur Radio transceiver project will 38 QEX July/August 2013 recognize the similarity of the HF7070 front end block diagram (see Figure 1) to the receiver in the CDG2000. Any reader who is interested in the detailed technical measure- ments for the HF7070 “proto2” receiver will find them on PA3AKEs web

13、site at http:/ martein.home.xs4all.nl/pa3ake/hmode/. Fundamental Design Issues in the Analog Signal Path Table 1 shows some of the key parameters of the receiver front end design. It is the job of the two-pole 45 MHz filter to provide some protection to the first IF amplifier for strong signals more

14、 than 10 kHz from the selected frequency. In an up-conversion radio the third order intercept (IP3) usually reduces significantly for off-channel signals within the bandwidth of its roofing filters. The close-in perfor- mance of the HF7070 has surprised quite a few people and Table 1 shows how this

15、excel- lent close-in IP3 performance is achieved. The values in the table of Net NF and Net IP3 are slightly better than the practical mea- surements on the “proto 2” receiver. In the table, the noise figure (NF) of the receiver at the antenna is 11 dB. This gives a noise floor of 129 dBm in a 2.4 k

16、Hz bandwidth. Together with a Net IP3 of 24.5 dBm the result is an in-band dynamic range of 102 dB. The practical results are NF 12 dB and an IP3 of 19 dBm, giving an in-band IP3 dynamic range of 97 dB. It is necessary for the analog front end to Table 1 Key Parameters of the HF7070 Front End Stage

17、Net Stage Net Stage Net Gain Gain IP3 IP3 NF NF dB dB dBm dBm dB dB Antenna low pass filter 2 2 45 24.5 2 11.1 First H-Mode mixer 5 7 45 22.5 5 9.1 2 pole 45 MHz filter 1.5 8.5 24 16 1.5 4.1 First 45 MHz amp 10 1.5 40 26 1.3 2.6 4 pole 45 MHz filter 2 0.5 24 24 2 7.7 Second 45 MHz amp 10 9.5 40 34 1

18、.3 5.7 Second H-Mode mixer 5 4.5 45 29 8 14.02 44 kHz amp 18 22.5 60 47 6 6.02 25 Bit ADC 0 22.5 50 47 4 4 Figure 1 The HF7070 front end block diagram with Out of Band IP3, Net Gain, State Gain and Noise figure indicated for various stages. have enough gain so that noise from the ana- log stages dom

19、inates the quantization noise from the ADC in the digital signal fed to the DSP. This gives a smooth, audible transition from noise to signal. Referring to Table 1, in-band IP3 is lim- ited by the 4-pole roofing filter. As the net IP3 requirement increases, as the signal is amplified the linearity w

20、ill be degraded if the net value exceeds the stage IP3. This does not happen with the HF7070, but it is interesting to note that the 44 kHz amplifier has an IP3 output of 60 dBm. A paradox is that a bit of signal path loss at the right place is a design virtue. The IP3 dynamic range at a 100 Hz test

21、 tone spac- ing in a 2.4 kHz bandwidth is 97 dB. The Yaesu FTDX-5000 has an IP3 of 8.5 dBm within the bandwidth of its 9 MHz roofing filters and a NF of 17 dB, which yields a dynamic range of 77 dB. So you can see why people with knowledge of receiver design are impressed with the close-in performan

22、ce of the HF7070. The First H-Mode Mixer The experimental 7070 front end that I gave to John used the Fairchild FST3125 fast bus switch in the mixer. An important result from Marteins experimental work was that the more recent Fairchild FSA3157 actually proved to be the best switch for use in H-Mode

23、 mixers. This is a SPDT switch with a 0.5 ns break-before-make action. This SPDT switch is ideal for being driven by a fundamental frequency squarer because it is not necessary for the drive logic to generate a complement signal, as was the case with the 45 kHz 35 MHz Low Pass Filter 4 H-Mode Mixer

24、H-Mode Mixer 25 Bit ADC TCXO 44.9555 MHZ 2-pole 15 kHz 45 MHz Filter Fundamental Frequency Squarer Fundamental Frequency Squarer Double-tank Oscillator Voltage Control Phase Detector 4-pole 15 kHz 45 MHz Filter 10 A A 10 A To DSP System LO 44.9555 MHz LO 45 to 75 MHz VR DDS Chip Control Set Frequenc

25、y 11.25 to 18.75 MHz ADC and DSP Clocks QX1305-Horrabin01 Analog Signal Path Frequency Synthesizer Out of Band IP3 at 50 kHz (dBm) 454338 Net Gain (dB)-2-7-8.5+1.5-0.5 +9.5+4.5 +22.5 Stage Gain (dB) -2-5-1.5+10-2+10-5+18 Noise Figure (dB) 11.19.14.12.67.75.714.026.024.0 QEX July/August 2013 39 FST31

26、25. The choice of Mini-Circuits trans- formers used in this mixer gives the radio an out-of-band IP3 of 45 dBm at a 50 kHz test tone spacing with high sensitivity down to an input frequency of 10 kHz. 15 kHz Bandwidth 2-Pole 45 MHz Crystal Filter Two of these filters are connected via quadrature hyb

27、rids to terminate the mixer. The use of quadrature hybrids with two iden- tical filters always presents a nominal 50 termination to the mixer, even when indi- vidual filters present a reactive load. A design goal of the HF7070 was to build a sensitive receiver without a preamplifier before the first

28、 H-Mode mixer. Because the HF7070 is a general coverage multimode receiver, it was always our intention to use a 15 kHz bandwidth roofing filter system to accommodate FM and DRM signals. Another reason for this is that narrower bandwidth crystal filters have lower design impedances, which would give

29、 a greater insertion loss and therefore require a pream- plifier before the first mixer. This filter and its surrounding circuitry introduce a loss of only 1.5 dB. The First 45 MHz Amplifier This amplifier drives the second roof- ing filter and it is the input IP3 of this filter (26 dBm) that limits

30、 the in-band performance of the radio. The amplifier is a 45 MHz version of the 4 J310 amplifier designed by Bill Carver, W7AAZ, as used in the CDG2000 trans- ceiver. The noise figure of this amplifier is 1.3 dB. Its output IP3 of 40 dBm makes it a particularly important building block in the HF7070

31、 to satisfy the requirement of having a sensitive, linear receiver without a pream- plifier before the first H-Mode mixer. This amplifier uses source gate feedback, which gives excellent reverse isolation so that its input impedance is not affected by its output driving a crystal filter, which in it

32、s transition region can present a reactive load. 15 kHz Bandwidth 4-Pole 45 MHz Crystal Filter This filter has an in-band IP3 at its input of 26 dBm and this ultimately controls the in-band IP3 of the radio. Some Amateur Radio equipment manufacturers offer nar- rower bandwidth VHF roofing filters fo

33、r their radios. You dont want this for two reasons: (1) the insertion loss increases and (2) the in-band IP3 for a narrower design bandwidth with the same quality of quartz crystals gets worse at roughly 6 dB per octave. This effect was discovered by PA3AKE when he was designing his 9 MHz roofing fi

34、lters with the fabulously linear crystals supplied by the German firm Quarztechnik. So what you really need in an up-con- version radio is a wide roofing filter, but the linearity of following circuitry must not degrade the in-band IP3 of the crystal filter. This seems to be a problem area with many

35、 transceiver designs. The second 45 MHz amplifier is similar to the first amplifier and uses the 4 J310. Its output IP3 is 6 dB higher than the net IP3 at that point. The second H-Mode mixer gives 115 dB rejection of the image because of the stop band of 6 poles of roofing filter, so an image reject

36、ion mixer is not required. This mixer gives a push-pull output centered on 44 kHz. The 44 kHz Amplifier John designed this amplifier with a noise figure of 6 dB, a gain of 18 dB and an output IP3 at 60 dBm. A point worth noting is that it is easier to get low-noise, high-IP3 gain at 45 MHz than at a

37、udio frequencies. This was an important consideration for the gain distribution of the front end. There is also a potential problem for the unwary that may affect other designs: many operational ampli- fiers have poor IP3 characteristics. You get good IP3 results at high signal levels, but the IP3 t

38、ones do not fall away as the signal amplitude is reduced; they remain at 60 dB down. There were a few operational amplifiers that gave the correct IP3 behavior. The reason for this problem may be due to crossover distortion in the output stage of most operational amplifiers. John was aware of this p

39、roblem when he designed the 44 kHz amplifier. It is interesting to note that the new ICOM IC-7410 with its 36 kHz IF appears to use a version of the TL074 that is known to have the IP3 problem. Apart from the front panel board, the radio has an analog board and a digital board. After some discussion

40、 the 25 bit ADC went on the digital board. The second H-Mode mixer makes the transition from 45 MHz to 44 kHz and it gives a push-pull balanced out- put. This is followed by a balanced amplifier at 44 kHz located on the analog board whose outputs are connected by a short length of strip cable from t

41、he analog board to the bal- anced inputs of the 25 bit ADC located on the digital board. Whatever common mode noise is present due to the strip cable connection between the two boards is well within the common mode rejection of the ADC. The 25 Bit ADC The radio uses a top-of-the-range, stereo, 24 bi

42、t audio ADC with both channels driven and the signals digitally added giving a theo- retical 25 bit performance, It is worth remem- bering that the ADC is effectively the main gain block. An estimate of its noise figure Further Developments The manufacture of the ten HF7070 production prototypes has

43、 been funded by a British company owned by a radio ama- teur. He was happy to learn that this article was appearing in QEX, but he asked that neither he nor his company be mentioned. In view of this it seems unlikely there will be a significant production run because he is concerned that there would

44、 be too much competition from SDRs. Collaboration between British and American electronic engineers during World War II resulted in some great design work. The thing to remember about the HF7070 is that although it has been designed by a British electronic engineer, all the components that really ma

45、tter, apart from the 45 MHz monolithic filters with their excellent IP3s, were designed in the USA. At least in that sense, history is repeating itself. It would have been more cost effective to use an MMIC in the 45 MHz IF strip, but nothing was available that met the technical requirements. That h

46、as changed recently, however. The American firm Mini-Circuits has introduced the PHA-1+ and the dual- matched version known as the PHA-11+. Although they were designed for microwave applications, the noise figure (NF) and IP3 out is at its best between 45 MHz and 100 MHz, making them useful in IF am

47、plifiers for up-conversion radios. Typical performance at 45 MHz is: NF 2.2 dB: gain 18 dB: and IP3 out 42 dBm. The vector network analyzer designed by Paul Kiciak, N2PK, together with the Windows software for the VNA written by Dave Robert, G8KBB, was used to measure the PHA-1 input impedance at 45

48、 MHz, which is 80 in parallel with 25 pF. The best way to use this part in the existing HF7070 IF strip is to use two in parallel with an output attenuator to replace the first 4 J310 amplifier, and one to replace the second amplifier, again with an output attenuator. A change to the HF7070 front en

49、d architecture by having all 6 poles of roof- ing filter connected via quadrature hybrids immediately after the first H-Mode mixer could, in principle, further increase close- in receiver dynamic range. Obviously, careful shielding around the filters is required. All the mechanical and electronic components for this experiment have been obtained and I hope to report the results in a future QEX article. From a practical printed circuit point of view, rather than use the matched pair the thermal affect on amplifier noise figure would have been reduced if two indivi