Abstract:
A device can include a radio frequency (RF) signal input, a local oscillator (LO) signal input, a mixer to receive the RF signal and LO signal and translate a frequency of the RF signal based on the LO signal, a strobe pulsing component to provide a timed strobe pulse, and a second mixer to receive a leakage signal, LO signal, and timed strobe pulse, and also to translate a frequency of the leakage signal to baseband. The device can also include a coupling component configured to allow the leakage signal to pass between the mixers. An output signal output can provide a measured value of the leakage signal.

Description:
BACKGROUND 
     Certain types of intermediate frequency (IF) receivers, such as a zero-IF type receiver, generally include a radio frequency (RF) converter having a local oscillator (LO) frequency that is near the center of the RF input bandwidth. For such a receiver, because the LO and RF frequencies may be equal or nearly equal, it is important that the LO signal leakage out of the input of the receiver be small. However, this is difficult to achieve because the LO signal generally cannot be separated from the RF signal by filtering. 
     At the RF converter, the LO signal typically has a significant amplitude, and mixers usually have limited isolation from the LO port to other ports, e.g., RF and IF ports. Isolation in the mixer can be maximized by applying direct current (DC) offset current into the IF port of the mixer, and a leakage detector can be used to measure the leakage signal as part of a feedback control loop which minimizes the leakage amplitude. 
     To date, various types of circuits have been developed to maintain low amplitudes of LO leakage from the RF input. While each idea may have some benefits, there are also a number of associated problems. Generally, there is a detector on the RF port side of the mixer to measure leakage LO power, and the detected amplitude may be used to form correction signals through a feedback control loop. 
       FIG. 1  illustrates an example of a presently-used implementation  100  of a LO leakage detector. Indeed, the illustrated circuitry represents a particularly popular implementation of a leakage detector. In the example, there are two input ports (an RF input port  102  and a LO input port  104 ) and a mixer  112  for providing an output signal at an output signal port  114 . A directional coupler  106  in the RF path transfers a portion of the reverse-travelling LO leakage signal (F LO ) to a wideband detector  108 . The detected output is measured by way of an analog-to-digital converter (ADC)  110 . 
     While the implementation illustrated by  FIG. 1  has certain advantages (e.g., the leakage LO signal is usually detected and the feedback control may remain engaged), the LO leakage signal is generally lost in thermal noise of the wideband detector  108  output, and correction does not typically work to certain low amplitudes. 
     Accordingly, there remains a need for improved signal detection circuits and devices having an LO input. 
     SUMMARY 
     Embodiments of the disclosed technology generally include systems and devices for improved local oscillator (LO) leakage detection. Embodiments generally pertain to the detector-part of a control loop and advantageously seek to detect a low-amplitude signal with adequate noise margin in a short amount of time. Certain embodiments may include a wideband strobe-pulsed detector having a short detection time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a presently-used implementation of a leakage detector. 
         FIG. 2  illustrates a first implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
         FIG. 3  illustrates a second implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
         FIG. 4  illustrates a third implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
         FIG. 5  illustrates a fourth implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
         FIG. 6  illustrates a fifth implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
         FIG. 7  illustrates a sixth implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
         FIG. 8  illustrates a seventh implementation of a leakage detector in accordance with certain embodiments of the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the disclosed technology generally pertain to fast signal detection in certain circuits and devices. Signal-to-noise ratios for a given detection speed are generally significantly better than those of presently-used implementations. Embodiments generally include a leakage detector in a local-oscillator-leakage-nulling circuit, though one having ordinary skill in the art will appreciate that other applications are feasible, such as implementations for reducing spurious signals in a frequency synthesizer. 
     Implementations of the disclosed technology generally use switches to directly connect the local oscillator (LO) leakage to the leakage detector while terminating the radio frequency (RF) input. The signal-to-noise ratio (SNR) of the leakage detector is generally better than with implementations in which a directional coupler is used. During the leakage detection time, the receiver is typically not active. 
     In order to keep from modifying the leakage amplitude, the impedance seen at the RF port of a mixer (e.g., at F LO ) should—be kept equivalent between leakage detection and normal RF conversion. This may be ensured with help from an RF attenuator, e.g., in cases where the RF input amplitude is high, or with a preamplifier, e.g., in cases where the RF input amplitude is low, or with a directional coupler. 
       FIG. 2  illustrates a first implementation  200  of a leakage detector, in this case a chopper-modulated leakage detector. In the example, there are two input ports (an RF input port  202  and a LO input port  204 ), and a first mixer  212  for providing an output signal, e.g., an IF signal, at an output signal port  214 . A second mixer  216  receives a copy of the LO input that is modulated by a low-RF chopping signal  208  by way of a low-frequency rate RF switch  210  or, alternatively, a mixer. 
     The illustrated implementation  200  uses discontinuous sampling of the leakage signal (F LO ). That is, when the leakage signal is measured, the RF input  202  is switched to a termination and the leakage signal is switched into the leakage detector by way of a first switch  206 . When the chopper signal  208  is turned on, it is mixed with the copy of the LO signal by the second mixer  216 . The leakage signal is then converted by the second mixer  216  to a narrow-band spectrum about the chop frequency, and that signal is subsequently digitized by an ADC  220 . This implementation  200  may work particularly well for good dynamic range on the leakage detection. 
     Because of the LO modulation, this method can extract the LO in the presence of a wideband RF signal or broadband noise, eliminating the need for switch  206  and/or enabling the use of a directional coupler, such as the directional coupler  106  of  FIG. 1 . 
       FIG. 3  illustrates a second implementation  300  of a leakage detector, in this case a strobe-pulsed leakage detector with envelope detection, in accordance with certain embodiments of the disclosed technology. This second implementation  300  is similar to the first implementation  200  illustrated by  FIG. 2  in that the present implementation  300  includes two input ports (an RF input port  302  and an LO input port  304 ), a first mixer  312  for providing an output signal, e.g., an intermediate frequency (IF) signal, at an output signal port  314 , and a chopper switch  310 . 
     Unlike the first implementation  200 , however, the chopper signal  208  has been replaced by a strobe pulse signal  308  in the presently illustrated implementation  300 . Also, as noted above, the present implementation  300  includes an envelope detector  320 . 
     The strobe pulse signal  308  width may be set to satisfy timing requirements of the chopper switch  310  plus some hold time for a stable amplitude, for example. The chopper output spectrum is generally dense about the leakage signal (F LO ). If the chopper switch  310  rise and fall times are much faster than the strobe pulse  308  width, the output spectrum is generally sin x/x. If the pulse width is not much wider than tr+tf, then output spectrum is more similar to a raised-cosine. Most of the power resides between the first nulls at f=1/T (square pulse case) where T is the width of the strobe pulse  308 . 
     A second mixer  316  may combine the strobed LO signal with the leakage signal received from a leakage switch  306 . The output spectrum is generally dense, mostly residing in the DC-to-1/T spectrum. Due to the non-linearity of the second mixer  316 , the mixer output spectrum is generally not the same shape as that of the input, but most of the power resides in the bandwidth from DC to f=1/T. 
     An optional low-pass filter (LPF)  318  may be used to band-limit what the detector measures to preserve a good SNR. The bandwidth should generally be set to f=1/T (or a nearby cutoff frequency) and, to avoid DC errors, a high-pass filter (not shown) can also be used, so long as the high-pass cutoff is sufficiently low in frequency. 
     An analog-to-digital converter (ADC)  322  may acquire a single sample when the output of the envelope detector  320  is at steady-state. The time constant of the detector does not need to be faster than the filter that precedes it. In fact, the detector itself may be the device that accomplishes low-pass filtering. 
     In certain implementations, the chopper switch  310  may have rise and fall times of 20 nanoseconds. The strobe pulse  308  width may be 60 nanoseconds, and the detector bandwidth may be 16 MHz. In such embodiments, the leakage detection amplitude may be measured below −90 dBm, and the detection time interval may be less than 200 nanoseconds. 
       FIG. 4  illustrates a third implementation  400  of a leakage detector, in this case a strobe-pulsed leakage detector with ADC detection, in accordance with certain embodiments of the disclosed technology. This third implementation  400  is similar to the second implementation  300  illustrated by  FIG. 3  in that the present implementation  400  includes two input ports (an RF input port  402  and an LO input port  404 ), a first mixer  412  for providing an output signal at an output signal port  414 , a leakage switch  406 , a strobe pulse signal  408 , a chopper switch  410 , and a leakage mixer  416 . 
     Unlike the second implementation  300 , however, the envelope detector  320  has been removed and the ADC  422  digitizes the output waveform of the LPF  418  in the presently illustrated implementation  400 . In the example, only one ADC sample, aligned in time with the strobe pulse, is necessary; But a few ADC samples taken during the strobe pulse time is beneficial for noise suppression. 
     At the leakage-detector mixer  416 , during the switch-on time, the leakage F LO  is present at both inputs. The phase separation of the chopped LO and the leakage LO signals should generally be such that the mixing does not result in a near-zero output amplitude. 
       FIG. 5  illustrates a fourth implementation  500  of a leakage detector, in this case a strobe-pulsed leakage detector using quadrature detection, in accordance with certain embodiments of the disclosed technology. This fourth implementation  500  is similar to the second and third implementations  300  and  400  illustrated by  FIGS. 3 and 4 , respectively, in that the present implementation  500  includes two input ports (an RF input port  502  and an LO input port  504 ), a first mixer  512  for providing an output signal at an output signal port  514 , a leakage switch  506 , a strobe pulse signal  508 , and a chopper switch  510 . 
     Unlike the second and third implementations  300  and  400 , however, the presently illustrated implementation  500  has two leakage mixers  516  and  517  and corresponding LPFs  518  and  519 , respectively. The leakage conversion is split into two channels, where the LO signal to one channel is phase-shifted from the other channel by 90 degrees. Also, two ADCs  522  (Q) and  523  (I) are used to digitize the output waveforms from the filters  518  and  519 , respectively. Thus, the leakage conversion may be performed using in-phase and quadrature channels, and the detected amplitude is generally the root-sum-of-squares of the I and Q values from the ADCs  523  and  522 , respectively. 
     In some microwave receivers, it may not be practicable to expect reasonable impedance matching at the mixer RF port when the input switches are in the “detect” position compared to “through.” In such situations, a directional coupler may be integrated with the strobe-pulsed receiver. The strobed receiver may thus keep the detected noise bandwidth small, and the coupler may ensure constant mixer RF port matching. 
       FIG. 6  illustrates an example of such a strobe-pulsed leakage detector using quadrature detection and directional coupler. This fifth implementation  600  is similar to the fourth implementation  500  illustrated by  FIG. 5  in that the present implementation  600  includes two input ports (an RF input port  602  and an LO input port  604 ), a first mixer  612  for providing an output signal at an output signal port  614 , a leakage switch  606 , a strobe pulse signal  608 , a chopper switch  610 , two leakage mixers  616  and  617 , two LPFs  618  and  619 , and two ADCs  622  and  623 . 
     Unlike the fourth implementation  500 , however, the leakage switch  506  has been removed and replaced with a directional coupler  606  in the presently illustrated implementation  600 . This arrangement may be particularly advantageous in certain situations, e.g., where the IF is non-zero and where there is enough separation such that leakage can be filtered from the RF signal in the LPFs  618  and  619  or in digital filters after the ADCs  622  and  623 . 
       FIG. 7  illustrates a sixth implementation  700  of a leakage detector, in this case a strobe-pulsed leakage detector for detecting leaking in the IF path, in accordance with certain embodiments of the disclosed technology. This sixth implementation  700  is similar to the fifth implementation  600  illustrated by  FIG. 6  in that the present implementation  700  includes two input ports (an RF input port  702  and an LO input port  704 ), a first mixer  712  for providing an output signal at an output signal port  714 , a leakage switch  706 , a strobe pulse signal  708 , a chopper switch  710 , two leakage mixers  716  and  717 , two LPFs  718  and  719 , and two ADCs  722  (I) and  723  (Q). 
     Unlike the fifth implementation  600 , however, the leakage switch  706  is located at the IF side of the circuitry. This arrangement may be particularly advantageous in certain situations, e.g., where the IF frequency has a spectrum that is close to that of the LO frequency. In the example, leakage may be measured at a time interval when the RF is disconnected from the leakage mixer  712 , which may be accomplished by way of another switch  707 . What is measured is fed back into LO-nulling circuitry (not shown) such as DC bias current driving the mixer diodes. 
       FIG. 8  illustrates a seventh implementation  800  of a leakage detector in accordance with certain embodiments of the disclosed technology. This seventh implementation  800  is similar to the sixth implementation  700  illustrated by  FIG. 7  in that the seventh implementation  800  includes two input ports (an RF input port  802  and an LO input port  804 ), a first mixer  812  for providing an output signal at an output signal port  814 , a leakage switch  806 , a strobe pulse signal  808 , a chopper switch  810 , two leakage mixers  816  and  817 , two LPFs  818  and  819 , and two ADCs  822  and  823 . 
     Unlike the sixth implementation  700 , however, the leakage switch  706  has been removed and replaced with a directional coupler  806  in the presently illustrated implementation  800 . In this arrangement, the leakage may always be measured but might not be separable from RF signals at certain low or near-zero frequencies. In such cases, however, the leakage may be measured at certain time intervals, e.g., when such signals are not present at the transmitting source or sources. 
     Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments may be modified in arrangement and detail without departing from such principles, and may be combined in any desired manner. And although the foregoing discussion has focused on particular embodiments, other configurations are contemplated. In particular, even though expressions such as “according to an embodiment of the invention” or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. 
     Consequently, in view of the wide variety of permutations to the embodiments described herein, this detailed description and accompanying material is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.