Tweak LO dithering for better performance

grc/PicoSDR-WBFM.grc

@@ -205,7 +205,7 @@ blocks:
     ant8: ''
     ant9: ''
     args: '"rtl_tcp"'
-    bb_gain0: '20'
+    bb_gain0: '0'
     bb_gain1: '20'
     bb_gain10: '20'
     bb_gain11: '20'
@@ -374,7 +374,7 @@ blocks:
     freq7: 100e6
     freq8: 100e6
     freq9: 100e6
-    gain0: '10'
+    gain0: '0'
     gain1: '10'
     gain10: '10'
     gain11: '10'
@@ -438,7 +438,7 @@ blocks:
     gain_mode7: 'False'
     gain_mode8: 'False'
     gain_mode9: 'False'
-    if_gain0: '20'
+    if_gain0: '0'
     if_gain1: '20'
     if_gain10: '20'
     if_gain11: '20'

grc/PicoSDR.grc

@@ -45,11 +45,23 @@ blocks:
     coordinate: [168, 8.0]
     rotation: 0
     state: enabled
-- name: samp_rate
+- name: decimation
   id: variable
   parameters:
     comment: ''
-    value: '50_000'
+    value: '4'
+  states:
+    bus_sink: false
+    bus_source: false
+    bus_structure: null
+    coordinate: [352, 8.0]
+    rotation: 0
+    state: enabled
+- name: rf_rate
+  id: variable
+  parameters:
+    comment: ''
+    value: '200_000'
   states:
     bus_sink: false
     bus_source: false
@@ -57,6 +69,38 @@ blocks:
     coordinate: [264, 8.0]
     rotation: 0
     state: enabled
+- name: samp_rate
+  id: variable
+  parameters:
+    comment: ''
+    value: rf_rate // decimation
+  states:
+    bus_sink: false
+    bus_source: false
+    bus_structure: null
+    coordinate: [456, 8.0]
+    rotation: 0
+    state: enabled
+- name: analog_agc_xx_0
+  id: analog_agc_xx
+  parameters:
+    affinity: ''
+    alias: ''
+    comment: ''
+    gain: '1.0'
+    max_gain: '65536'
+    maxoutbuf: '0'
+    minoutbuf: '0'
+    rate: 1e-4
+    reference: '0.7'
+    type: complex
+  states:
+    bus_sink: false
+    bus_source: false
+    bus_structure: null
+    coordinate: [360, 312.0]
+    rotation: 0
+    state: enabled
 - name: analog_quadrature_demod_cf_0
   id: analog_quadrature_demod_cf
   parameters:
@@ -70,7 +114,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [640, 536.0]
+    coordinate: [768, 616.0]
     rotation: 0
     state: true
 - name: blocks_message_debug_0
@@ -85,7 +129,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [928, 32.0]
+    coordinate: [1056, 112.0]
     rotation: 0
     state: true
 - name: blocks_probe_rate_0
@@ -105,7 +149,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [640, 40.0]
+    coordinate: [768, 120.0]
     rotation: 0
     state: true
 - name: digital_costas_loop_cc_0
@@ -123,9 +167,33 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [640, 344.0]
+    coordinate: [768, 424.0]
     rotation: 0
     state: true
+- name: low_pass_filter_0
+  id: low_pass_filter
+  parameters:
+    affinity: ''
+    alias: ''
+    beta: '6.76'
+    comment: ''
+    cutoff_freq: samp_rate / 8
+    decim: decimation
+    gain: '1'
+    interp: '1'
+    maxoutbuf: '0'
+    minoutbuf: '0'
+    samp_rate: rf_rate
+    type: fir_filter_ccf
+    width: samp_rate / 8
+    win: window.WIN_HAMMING
+  states:
+    bus_sink: false
+    bus_source: false
+    bus_structure: null
+    coordinate: [520, 284.0]
+    rotation: 0
+    state: enabled
 - name: osmosdr_source_0
   id: osmosdr_source
   parameters:
@@ -164,7 +232,7 @@ blocks:
     ant8: ''
     ant9: ''
     args: '"rtl_tcp"'
-    bb_gain0: '20'
+    bb_gain0: '0'
     bb_gain1: '20'
     bb_gain10: '20'
     bb_gain11: '20'
@@ -333,7 +401,7 @@ blocks:
     freq7: 100e6
     freq8: 100e6
     freq9: 100e6
-    gain0: '10'
+    gain0: '0'
     gain1: '10'
     gain10: '10'
     gain11: '10'
@@ -397,7 +465,7 @@ blocks:
     gain_mode7: 'False'
     gain_mode8: 'False'
     gain_mode9: 'False'
-    if_gain0: '20'
+    if_gain0: '0'
     if_gain1: '20'
     if_gain10: '20'
     if_gain11: '20'
@@ -465,7 +533,7 @@ blocks:
     minoutbuf: '0'
     nchan: '1'
     num_mboards: '1'
-    sample_rate: samp_rate
+    sample_rate: rf_rate
     sync: sync
     time_source0: ''
     time_source1: ''
@@ -480,7 +548,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [152, 164.0]
+    coordinate: [104, 244.0]
     rotation: 0
     state: enabled
 - name: qtgui_const_sink_x_0
@@ -572,7 +640,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [928, 328.0]
+    coordinate: [1056, 408.0]
     rotation: 0
     state: true
 - name: qtgui_time_sink_x_0
@@ -669,7 +737,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [640, 232.0]
+    coordinate: [768, 312.0]
     rotation: 0
     state: true
 - name: qtgui_time_sink_x_0_0
@@ -766,7 +834,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [928, 512.0]
+    coordinate: [1056, 592.0]
     rotation: 0
     state: true
 - name: qtgui_time_sink_x_0_1
@@ -863,7 +931,7 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [928, 400.0]
+    coordinate: [1056, 480.0]
     rotation: 0
     state: true
 - name: qtgui_waterfall_sink_x_0_0
@@ -924,20 +992,22 @@ blocks:
     bus_sink: false
     bus_source: false
     bus_structure: null
-    coordinate: [640, 128.0]
+    coordinate: [768, 208.0]
     rotation: 0
     state: true
 
 connections:
+- [analog_agc_xx_0, '0', low_pass_filter_0, '0']
 - [analog_quadrature_demod_cf_0, '0', qtgui_time_sink_x_0_0, '0']
 - [blocks_probe_rate_0, rate, blocks_message_debug_0, print]
 - [digital_costas_loop_cc_0, '0', qtgui_const_sink_x_0, '0']
 - [digital_costas_loop_cc_0, '0', qtgui_time_sink_x_0_1, '0']
-- [osmosdr_source_0, '0', analog_quadrature_demod_cf_0, '0']
-- [osmosdr_source_0, '0', blocks_probe_rate_0, '0']
-- [osmosdr_source_0, '0', digital_costas_loop_cc_0, '0']
-- [osmosdr_source_0, '0', qtgui_time_sink_x_0, '0']
-- [osmosdr_source_0, '0', qtgui_waterfall_sink_x_0_0, '0']
+- [low_pass_filter_0, '0', analog_quadrature_demod_cf_0, '0']
+- [low_pass_filter_0, '0', blocks_probe_rate_0, '0']
+- [low_pass_filter_0, '0', digital_costas_loop_cc_0, '0']
+- [low_pass_filter_0, '0', qtgui_time_sink_x_0, '0']
+- [low_pass_filter_0, '0', qtgui_waterfall_sink_x_0_0, '0']
+- [osmosdr_source_0, '0', analog_agc_xx_0, '0']
 
 metadata:
   file_format: 1

src/main.c

@@ -251,12 +251,11 @@ static void lo_generate(uint32_t *buf, double freq, uint32_t phase)
 {
 	freq_step = STEP_BASE * freq;
 
-	unsigned down = 4 + __builtin_clz(freq_step);
+	unsigned down = 2 + __builtin_clz(freq_step);
 
 	for (size_t i = 0; i < LO_WORDS; i++) {
 		uint32_t bits = 0;
-		int shift = (rnd_next() >> down) - (rnd_next() >> down) + (rnd_next() >> down) -
-			    (rnd_next() >> down);
+		int shift = (rnd_next() >> down) - (rnd_next() >> down);
 
 		for (int j = 0; j < 32; j++) {
 			bits |= (phase + shift) >> 31;
@@ -272,13 +271,12 @@ static void lo_tweak(uint32_t *buf, uint32_t phase)
 {
 	static size_t i = 0;
 	uint32_t bits = 0;
-	unsigned down = 4 + __builtin_clz(freq_step);
+	unsigned down = 2 + __builtin_clz(freq_step);
 
 	phase += freq_step * i * 32;
 
 	for (int j = 0; j < 32; j++) {
-		int shift = (rnd_next() >> down) - (rnd_next() >> down) + (rnd_next() >> down) -
-			    (rnd_next() >> down);
+		int shift = (rnd_next() >> down) - (rnd_next() >> down);
 		bits |= (phase + shift) >> 31;
 		bits <<= 1;
 		phase += freq_step;
@@ -602,8 +600,10 @@ static void rf_rx(void)
 			iq_queue_pos = (iq_queue_pos + 1) & (IQ_QUEUE_LEN - 1);
 		}
 
-		lo_tweak(lo_cos, COS_PHASE);
-		lo_tweak(lo_sin, SIN_PHASE);
+		for (int i = 0; i < 8; i++) {
+			lo_tweak(lo_cos, COS_PHASE);
+			lo_tweak(lo_sin, SIN_PHASE);
+		}
 	}
 }