Software Defied Radios Basic Desigi FARA Nov. 2017 1 / 71
Outliie ● Superhet Analog Radio Example for background ● Direct Conversion Receivers – Early Digital Implementations – Direct Down Conversion Digital Implementations ● Direct Conversion Transmitters – Direct Up Conversion Digital Implementations ● Example SDR Transceiers ● Speculation on Future SDRs 2 / 71
Backgrouid: K3 Multi-stage Superhet IF Out to P3 Noise Crystal BPFs X X Blanker BPFs 9-46 MHz 8.23 MHz VCO 2nd LO 15 kHz BW DAC/ LPF ADC DSP LPF/ Amp ● Even analog K3 uses Digital Signal Processing for noise blanking and limiting, AGC, amplifcation, fltering, and demodulation. ● K3 Panadapter, the P3, is really a SDR. 3 / 71
K3 Block Diagram 4 / 71
Backgrouid Aside: P3 is a SDR 5 / 71
Direct Coiversioi Reciever f BASE OUT = 0 Hz f BASE IN = N MHz RF Demodulation, Mixer Stage Panadapter, etc. f BASE IN Local Oscillator ● Single mixer stage translates a spectrum at RF down to 0 Hz. ● Simple structure is well matched for digital implementation, several implementation options. 6 / 71
Image Problem f BASE OUT = 0 Hz f BASE IN = N MHz RF X LPF f BASE IN Local Oscillator ● Input spectrum just below f BASE IN is also mapped to output. ● Sharp pre-mixer BPF to remove problem spectrum is difcult 7 / 71
Image Problem ● sin A x sin B = ½ (cos (A – B) – cos (A + B) ) ● sin (2 ( f BI + ) t) x sin (2 f BI t) = ½(cos (2 t) – cos (2 (2 f BI + ) t)) ● sin (2 ( f BI - ) t) x sin (2 f BI t) = ½(cos (-2 t) – cos (2 (2 f BI - ) t)) ● We can not distinguish between cos (-2 t) and cos (2 t) in the output. 8 / 71
Backup: Plot of Cosiie Symmetric around Y axis X=- /2 X= /2 9 / 71
Image Reject Architecture Incident X RF f BI + LO 0Hz-based 90 delay spectrum Quadrature 90 delay X ● Mixing with both cosine and sine provides enough information to distinguish between desired signal and undesired image . ● 2nd 90 delay should be frequency independent over desired freq range. Digital FIR can be close. 10 / 71
Processiig of sii(2 ( f BI + ) t) ● I: sin (2 ( f BI + ) t) x sin (2 f BI t) = ½(cos (2 t) – cos (2 (2 f BI + ) t)) ● Q: sin (2 ( f BI + ) t) x sin ((2 f BI t) - /2) = ½(cos ((2 t) + /2) – cos ((2 (2 f BI + ) t) - /2) ) ● Q delayed by /2: ½(cos (2 t) – cos ((2 (2 f BI + ) t) - ) ) ● I + (Q delayed): cos (2 t) 11 / 71
Processiig of sii(2 ( f BI - ) t) ● I: sin (2 ( f BI - ) t) x sin (2 f BI t) = ½(cos (2 t) – cos (2 (2 f BI - ) t)) ● Q: sin (2 ( f BI - ) t) x sin ((2 f BI t) - /2) = ½(cos ((2 t) - /2) – cos ((2 (2 f BI - ) t) - /2) ) ● Q delayed by /2: ½(cos ((2 t) - ) – cos ((2 (2 f BI + ) t) - ) ) ● I + (Q delayed): 0 12 / 71
Digital Implemeitatioi Where to digitize? I X RF f BI + LO 0Hz-based spectrum 90 delay Q X 90 delay > 16bits x (2x30) Msps ● In the early days, 2000?, low-cost ADCs could not support the complete 0-30 Mhz bandwidth. 13 / 71
Digital Implemeitatioi Early Geieratioi SDR Receivers I LPF ADC X RF Digital f BI LO Processing: Panadapter 90 delay Noise Filters Demodulation Q LPF ADC X Analog PC Sound PC Software PC Components Card Sound 2 x (16 to 18) bits x (44 to 192) ksps ● Flex SDR-1000, Softrock ● KX3: similar but all digital processing on-board 14 / 71
Softrock 15 / 71
Flex SDR 1000 16 / 71
Elecraft KX3 17 / 71
Early Geieratioi SDR Receivers Advaitages aid Issues I LPF ADC X RF Digital f BI LO Processing: Panadapter 90 delay Noise Filters Demodulation Q LPF ADC X Analog PC Sound PC Software PC Components Card Sound ● Simple Hardware with few components ● Hard to precisely match I & Q amplitude & phase, though digital comp for fxed diferences possible 18 / 71
Digital Implemeitatioi Better Cheap ADCs I X RF f BI + LPF ADC LO 90 delay Q X 90 delay > 16bits x (2x30) Msps ● ADCs good-enough now for entire HF bandwidth, though some issues which we will discuss later ● But can‘t get whole 1Gbps rate in/out of uP chip 19 / 71
Digital Implemeitatioi Direct Dowi Coiversioi Example: openHPSDR Hermes I LPF Decimate X RF Digital LPF ADC LO Processing: Panadapter 90 Noise Filters Demodulation Q LPF Decimate X RF Analog FPGA PC Software PC Sound 16 bits x 122.88 Msps 2 x 24 bits x 192 ksps ● FGPA DSP; anti-alias LPF I/Q, reduces sample rate to min needed for panadapter and demodulation 20 / 71
Backup: Decimate Alias Issue, /2 Decimate Example 21 / 71
Direct Dowi Coiversioi FPGA Processiig Example: openHPSDR Hermes CORDIC NCO I CIC CFIR X LPF & LPF, cosine decimate decimate by 320 by 2, & 16 bits x 2 x 24 bits x fix slope 122.88 Msps 192 ksps sine Q CIC CFIR X LPF & LPF, decimate decimate by 320 by 2, & fix slope 22 / 71
Direct Dowi Coiversioi FPGA Processiig (coitiiued) ● COordinate Rotation DIgital Computer uses just add, subtract, lookup, and bit shift successive approximation used in early calculators such as HP35 23 / 71
Direct Dowi Coiversioi FPGA Processiig (coitiiued) Cascaded Integrator-comb Filter used in Hermes + + - - Delay Delay Delay Delay ... ... 10 Integrate Stages Decimate 10 Comb Stages running at 122.88 Msps by 320 running at 384 ksps ● Does LPF, good at attenuating higher frequencies ● Does most of the decimation ● Uses only adds and pipeline delays ● But not fat in pass band 24 / 71
Direct Dowi Coiversioi FPGA Processiig (coitiiued) Compensating Finite Impulse Response Filter used in Hermes Delay Delay … 256 stages C 1 C 2 X X ADD ● Used with Blackman-Harris Window Function to get sharper roll-of. 100dB attenuation of aliases. ● Coefcients adjusted to fx passband response. ● Decimating by 2, only do multiplies every 2nd clk 25 / 71
Direct Dowi Coiversioi SDR Receivers/Traisceivers for Ham Baids (leveragiig cell phoie chips?) ● Perseus receiver ~2007 ● openHPSDR Mercury reciever ~2008 Hermes transceiver ~2010? ● Anan transceivers (based on openHPSDR) ~2014? ● Flex 6000 transceivers ~2014? ● Icom 7300, 7610, R8600 26 / 71
Direct Dowi Coiversioi Advaitages aid Issues ● Mixing and fltering done digitally. I/Q amplitude and phase are very accurately matched. Processing noise can be reduced to arbitrary levels. ● Performance of volume ADCs sufcient but not fantastic. – Wide band ADC SNR may only be ~77dBFS. Not a real issue since we use narrow bandwidths. ~3dB gain for each factor of 2 decimation. 27 / 71
Backup: Sigial-to-Noise Ratio 28 / 71
Backup: Oversampliig aid SNR F s /2 29 / 71
Direct Dowi Coiversioi Advaitages aid Issues (coitiiued) ● ADC performance (continued) – Higher resolution can be achieved by the oversampling/decimation process. 16-bit or even 14-bit ADCs can be good enough if they have sufcient linearity. 30 / 71
Backup: Small Sigial aid No Noise 31 / 71
Backup: Iicreasiig Resolutioi by Decimatioi 32 / 71
Backup: Iicreasiig Resolutioi by Decimatioi 33 / 71
Backup: Iicreasiig Resolutioi by Decimatioi 118.948 118.721 34 / 71
Direct Dowi Coiversioi Advaitages aid Issues (coitiiued) ● ADC performance (continued) – ADC SFDR, Spurious-Free Dynamic Range, roughly the ratio of a fundamental signal to the strongest resulting spurious signal in the output, may only be ~94dBc. I think that decimation process gain does not help since the spurious signal is at a fxed frequency. 35 / 71
Direct Dowi Coiversioi Advaitages aid Issues (coitiiued) ● ADC performance (continued) – The efective dynamic range of the ADC and the wide bandwidth into the ADC may allow ADC overload. Any signal in passband of the pre-ADC flter can cause overload. Issue can be limited to a particular band by a sharp pre- ADC BPF. For example Flex has put sharper BPFs for contest bands in their new 6600. 36 / 71
Direct Dowi Coiversioi Advaitages aid Issues (coitiiued) ● Performance/latency/FPGA size compromises that I don‘t completely understand in the flters. 37 / 71
Direct Dowi Coiversioi Advaitages aid Issues (coitiiued) ● Nonetheless the current crop of high-end SDR receivers perform as well or better than the best high-end analog receivers on conventional measurements such as Sherwood Engineering‘s dynamic range tests. 38 / 71
Sherwood List, page 1 39 / 71
Sherwood List, page 2 40 / 71
Sherwood List, page 5 41 / 71
Direct Coiversioi Traismitter SSB Exciter I X Audio USB f RF - LO 90 Q 90 X ● Pretty much reverse of receiver. 42 / 71
Digital Implemeitatioi Direct Up Coiversioi 43 / 71
opeiHPSDR Hermes Traisceiver 44 / 71
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