RTL2382U/R820T 搭配 RTL-SDR 的調校

ytsejam

很多人都會用 RTL2832U/R820T 的 USB 接收模組搭配 RTL-SDR / SDR# / GQRX / dump1090 之類的程式使用
用作 SDR 或是ADSB Receiver

接著大多數人都是從製作、調整天線做起，但很少有討論到調整設定來使收訊更穩定精準

以下分享一些經驗，希望可以幫助有興趣玩 SDR 的朋友


RTL2832U/R820T 或 R820T2 是市面上所能買到的最便宜的 USB 接收模組
但這種成本考量為優先的產品，通常其元件也是以低成本為主要考量



Cost down 若拿捏得好，倒不至於影響品質太多，但如上圖中央的晶振，似乎就是省到極致的選擇
而這會直接影響收訊的品質，因此若不加以調整，即使天線做得再好，整體使用上還是讓人不甚滿意

以下是我自己使用 GQRX 的範例



上圖中，我要收聽的頻率是 125.5 MHz ，台北飛航管制區 Taipei Control 海峽席航管頻道
可以注意到，若要能正常收聽，必須調整頻率誤差為 -3.9kHz ，若不加以調整，直接設定為 125.5 MHz ，就會收不到訊號

這正是因為 RTL2832U 上便宜的晶振的誤差所造成的
而幸運的是，這可以透過軟體加以修正

在 RTL-SDR 中，有一個工具是 rtl_test ，使用以下指令，可以幫助我們找出頻率誤差的 ppm 值
(-d 1 指定使用第二個 RTL2832U，-p 則是列出 ppm 誤差)

root@ADSB:~# rtl_test -d 1 -p
Found 2 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001
  1:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 1: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
[R82XX] PLL not locked!
Sampling at 2048000 S/s.
Reporting PPM error measurement every 10 seconds...
Press ^C after a few minutes.
Reading samples in async mode...
lost at least 28 bytes
real sample rate: 2048132 current PPM: 65 cumulative PPM: 65
real sample rate: 2048042 current PPM: 21 cumulative PPM: 42
real sample rate: 2048091 current PPM: 45 cumulative PPM: 43
real sample rate: 2048065 current PPM: 32 cumulative PPM: 40
real sample rate: 2048082 current PPM: 40 cumulative PPM: 40
real sample rate: 2048046 current PPM: 23 cumulative PPM: 37
real sample rate: 2048067 current PPM: 33 cumulative PPM: 37
real sample rate: 2048087 current PPM: 43 cumulative PPM: 37
real sample rate: 2048014 current PPM: 7 cumulative PPM: 34
real sample rate: 2048069 current PPM: 34 cumulative PPM: 34
real sample rate: 2048094 current PPM: 46 cumulative PPM: 35
real sample rate: 2048098 current PPM: 48 cumulative PPM: 36
real sample rate: 2048046 current PPM: 23 cumulative PPM: 35
real sample rate: 2048071 current PPM: 35 cumulative PPM: 35
real sample rate: 2048025 current PPM: 12 cumulative PPM: 34
real sample rate: 2048092 current PPM: 45 cumulative PPM: 34
real sample rate: 2048080 current PPM: 39 cumulative PPM: 35
real sample rate: 2048087 current PPM: 43 cumulative PPM: 35
real sample rate: 2048030 current PPM: 15 cumulative PPM: 34
real sample rate: 2048064 current PPM: 31 cumulative PPM: 34
real sample rate: 2047004 current PPM: -486 cumulative PPM: 9
real sample rate: 2049153 current PPM: 563 cumulative PPM: 34
real sample rate: 2048072 current PPM: 35 cumulative PPM: 34
real sample rate: 2048074 current PPM: 36 cumulative PPM: 34
real sample rate: 2048081 current PPM: 40 cumulative PPM: 34
real sample rate: 2048042 current PPM: 21 cumulative PPM: 34
real sample rate: 2048087 current PPM: 43 cumulative PPM: 34
real sample rate: 2048045 current PPM: 22 cumulative PPM: 34
real sample rate: 2048079 current PPM: 39 cumulative PPM: 34
real sample rate: 2048054 current PPM: 26 cumulative PPM: 34
real sample rate: 2048093 current PPM: 46 cumulative PPM: 34
real sample rate: 2048035 current PPM: 17 cumulative PPM: 34
real sample rate: 2048056 current PPM: 28 cumulative PPM: 33
real sample rate: 2048092 current PPM: 45 cumulative PPM: 34
real sample rate: 2048030 current PPM: 15 cumulative PPM: 33
real sample rate: 2048113 current PPM: 56 cumulative PPM: 34
real sample rate: 2048062 current PPM: 31 cumulative PPM: 34
real sample rate: 2048087 current PPM: 43 cumulative PPM: 34
real sample rate: 2048064 current PPM: 32 cumulative PPM: 34
real sample rate: 2048059 current PPM: 29 cumulative PPM: 34
real sample rate: 2048074 current PPM: 36 cumulative PPM: 34
real sample rate: 2048068 current PPM: 33 cumulative PPM: 34
real sample rate: 2046505 current PPM: -730 cumulative PPM: 16
real sample rate: 2049594 current PPM: 779 cumulative PPM: 33
real sample rate: 2048067 current PPM: 33 cumulative PPM: 33
real sample rate: 2048078 current PPM: 39 cumulative PPM: 34
real sample rate: 2048062 current PPM: 31 cumulative PPM: 33

執行上述命令後，rtl_test 會計算累計的誤差值，執行約 15 ~ 30 分鐘後，累計的 ppm 誤差也會趨於一個穩定值，
這個穩定值 (在本例中約為 33 ppm)，就可以用來修正 RTL2832u 的頻率誤差

因此我使用 33 ppm 為修正值，套用的 rtl_sdr / rtl_tcp 中：

# rtl_tcp -d 1 -a 0.0.0.0 -p 1234 -s 1000000 -g 40.2 -P 33 -n 200

上列中的 -P 33 即是修正頻率誤差為 33 ppm

修正後，執行 GQRX 的結果如下



可以注意到頻率誤差就被補正了

相同的原理，也可以應用的 ADSB 接收程式，如 dump1090 上，以下是我用來修正另一個 RTL2832U 的命令 (ppm=48)

# dump1090 --device-index 0 --interactive --net --no-fix --ppm 48

希望可以供大家參考

cuisrt

我是用這組

不知道是不是更精簡了,軟體我用SDR#接車天線聽效果普通,人聲不清楚都看他頻譜來微調到正確的地方,用AT-398也不好,後來用TH-F7E接數位電視天線 新能 NPE-4DXB效果實在棒人聲非常清晰

ytsejam

cuisrt 發表於 2015-11-12 11:49
我是用這組

不知道是不是更精簡了,軟體我用SDR#接車天線聽效果普通,人聲不清楚都看他頻譜來微調到正確的地 ...

您這組還不錯，晶振看起來是 SMD TCXO ，若精度夠，應該頻偏會比較小

目前計劃是要用外部 TCXO 晶振來取代原本的 Crystal

ytsejam

cuisrt 發表於 2015-11-12 11:49
我是用這組

不知道是不是更精簡了,軟體我用SDR#接車天線聽效果普通,人聲不清楚都看他頻譜來微調到正確的地 ...

今天要加裝 TCXO 時，檢查了一下RTL2832U 上的電路，上面確實有預留 SMD 的 Crystal 腳位，不過可惜不是 TCXO，只能用外掛 TCXO 的方式，不過好處是 RTL2832U 板子上都有微溫，至少有 30 度以上
把 TCXO 用另一張板子裝，也可以減少溫度的影響

ytsejam

上圖為裝上 TCXO 的樣子，模組上有 74HC14 做 Buffer ，所以單一顆 TCXO 可輸出三組 Clock
TCXO 是淘寶買的，規格號稱 0.5ppm，姑且信之，再差也要比原來的晶振要好

以下是 rtl_test 的輸出

root@ADSB:~# rtl_test -d 0 -p
Found 2 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000001
  1:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM
Found Rafael Micro R820T tuner
Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
[R82XX] PLL not locked!
Sampling at 2048000 S/s.
Reporting PPM error measurement every 10 seconds...
Press ^C after a few minutes.
Reading samples in async mode...
lost at least 200 bytes
real sample rate: 2048006 current PPM: 3 cumulative PPM: 3
real sample rate: 2047997 current PPM: -1 cumulative PPM: 1
real sample rate: 2047995 current PPM: -2 cumulative PPM: 0
real sample rate: 2048026 current PPM: 13 cumulative PPM: 3
real sample rate: 2047998 current PPM: -1 cumulative PPM: 2
real sample rate: 2047998 current PPM: -1 cumulative PPM: 2
real sample rate: 2048000 current PPM: 0 cumulative PPM: 2
real sample rate: 2047994 current PPM: -2 cumulative PPM: 1
real sample rate: 2047991 current PPM: -4 cumulative PPM: 1
real sample rate: 2048005 current PPM: 3 cumulative PPM: 1
real sample rate: 2047997 current PPM: -1 cumulative PPM: 1
real sample rate: 2047991 current PPM: -4 cumulative PPM: 0
real sample rate: 2048006 current PPM: 3 cumulative PPM: 0
real sample rate: 2047996 current PPM: -2 cumulative PPM: 0
real sample rate: 2048001 current PPM: 1 cumulative PPM: 0
real sample rate: 2047992 current PPM: -3 cumulative PPM: 0
real sample rate: 2048027 current PPM: 14 cumulative PPM: 1
real sample rate: 2047991 current PPM: -4 cumulative PPM: 1
real sample rate: 2047978 current PPM: -11 cumulative PPM: 0
real sample rate: 2048023 current PPM: 11 cumulative PPM: 1
real sample rate: 2047991 current PPM: -4 cumulative PPM: 0
real sample rate: 2048004 current PPM: 2 cumulative PPM: 0
real sample rate: 2047996 current PPM: -2 cumulative PPM: 0
real sample rate: 2047994 current PPM: -2 cumulative PPM: 0
real sample rate: 2047990 current PPM: -5 cumulative PPM: 0
real sample rate: 2048010 current PPM: 5 cumulative PPM: 0
real sample rate: 2047999 current PPM: 0 cumulative PPM: 0
real sample rate: 2047996 current PPM: -2 cumulative PPM: 0
real sample rate: 2048022 current PPM: 11 cumulative PPM: 0
real sample rate: 2047996 current PPM: -2 cumulative PPM: 0
real sample rate: 2047998 current PPM: -1 cumulative PPM: 0
real sample rate: 2048000 current PPM: 0 cumulative PPM: 0
real sample rate: 2047991 current PPM: -4 cumulative PPM: 0
real sample rate: 2048002 current PPM: 1 cumulative PPM: 0
real sample rate: 2048003 current PPM: 2 cumulative PPM: 0
real sample rate: 2047991 current PPM: -4 cumulative PPM: 0
real sample rate: 2048002 current PPM: 1 cumulative PPM: 0
real sample rate: 2047996 current PPM: -2 cumulative PPM: 0
real sample rate: 2048001 current PPM: 1 cumulative PPM: 0
real sample rate: 2047996 current PPM: -2 cumulative PPM: 0

可以看到頻率偏移幾乎沒有了

winchen

感動耶

真是太強了，可是 windows 版本可以調偏移嗎?

ytsejam

winchen 發表於 2015-11-25 17:09
感動耶

真是太強了，可是 windows 版本可以調偏移嗎?

看您所用的程式，小弟用的是 dump1090，支援 ppm 調整

bevistsai

ytsejam 發表於 2015-11-14 01:20
上圖為裝上 TCXO 的樣子，模組上有 74HC14 做 Buffer ，所以單一顆 TCXO 可輸出三組 Clock
TCXO 是淘寶買 ...

請問一下，這個TCXO模組哪買的到，要用什麼關鍵字搜尋?
謝謝~

ytsejam

bevistsai 發表於 2016-11-9 21:11
請問一下，這個TCXO模組哪買的到，要用什麼關鍵字搜尋?
謝謝~

這個是阿仁的 Pure Clock 模組，只用了中間的部分

http://goods.ruten.com.tw/item/show?21010099146496

bevistsai

ytsejam 發表於 2016-11-10 18:36
這個是阿仁的 Pure Clock 模組，只用了中間的部分

http://goods.ruten.com.tw/item/show?21010099146496 ...

非常感謝，原來是阿仁的，想說在淘寶找了老半天怎麼也找不到。